I would say Yes, especially since you specify "sole light source", and because flash is flash. But still as follows.
This much is true of any system brand:
Every flash picture is a double exposure, of the flash (which might be strong or weak), and of the ambient light (which might be strong or weak). Until we realize this, we don't know much about flash.
The camera system meter meters the ambient light, and the TTL flash system meters the flash preflash. There are two metering systems, and the one we see is only about ambient. Therefore, camera automation sets shutter speed and/or aperture for the ambient. Then the flash discovers it has to work into those settings. There may be better choices for flash though, and for the overall picture result. And of course camera Manual mode bypasses that automation, and we set the settings ourselves, as we desire them to be, for whatever reason we choose.
Shutter speed does Not affect the flash exposure, but of course, shutter speed does affect the ambient exposure. The ambient light is continuous light, affected by the shutter sample time. Flash is more instantaneous, it simply happens while the shutter is already open (but is obviously not affected by shutter speed). This allows use of shutter speed to control just the ambient.
The thing about camera M mode is that it is fixed however we set it, is always how we set it. Automation is not messing around with it, just because the camera or the subject moved. We set aperture for flash power level, and specifically, camera M mode allows us to set shutter speed any way we desire to control the ambient, while the TTL automation is controlling the flash exposure. I believe this is the same reason you speak of camera M mode with flash.
For example, in a studio situation, we likely choose to use a shutter speed near maximum sync speed, to keep out the orange ambient incandescent light. The fact ambient is no longer a factor certainly does not change the basics, instead it was due to our choice about control of ambient with shutter speed. We chose shutter speed to ignore ambient.
Re Nikon Vs Canon:
I have to think more things about flash have to be the same - flash is flash. However programs doing automation can be written any way desired. Manual mode bypasses much of that, and does what we set it to do.
If you also use manual flash mode, then it too has no automation, and does exactly what you specify.
But if you use TTL flash on either brand, then there are quirks of reflective metering (white subjects come out underexposed, black subjects come out over exposed).
I know nearly nothing about Canon, but I am aware of a couple of things.
One major difference is that on Nikon, Exposure Compensation affects
both ambient and flash. On Canon, it affects ambient only. Both also have Flash Compensation, which affects only flash. But Nikon considers flash to be exposure too. Both systems have pros and cons, and some of the newest Nikon models (D7100, D600) have a new option to isolate them (Canon style, so to speak) if you choose. On D90, exposure compensation always affects both.
Nikon TTL has two modes, called TTL and TTL BL (BL is balanced flash, reduced flash to not overexpose ambient).
Canon has two modes too, Average and Evaluative. Sources I consider very reliable and trustworthy (
http://neilvn.com/tangents/canon-ettl-flash-average-vs-evaluative/ for example) seem to think this does in fact compare to Nikons modes TTL and TTL BL,
in that one (TTL BL or Evaluative) takes background exposure into account when determining flash exposure.
And one (TTL or Average) does not. The flash comes ahead on without regard to ambient.
In this respect, Nikon manuals tend to have the quirk of calling ambient to be "background", and flash foreground to be "subject".
Both TTL BL or Evaluative are programs, which can be written to do any fool thing. But all either can do about flash exposure regarding ambient is to back off on the flash, so that it will not overexpose the ambient. If the ambient meter meters the ambient and provides correct exposure, and if the TTL flash meters the flash to provide correct exposure, that is of course two correct exposures (of the near subject), which add to 2x exposure, or one stop overexposure of the subject. Every time. And this is what TTL mode will do (having no regard for bright ambient), so we know we have to provide around -2 EV flash compensation in that case (bright metered ambient). But TTL BL (balanced flash) will back off itself about -2 EV (fill flash in bright sunlight), and the correct flash compensation then might be 0 EV. Indoors, we hope neither backs off, but Nikon TTL BL tends to, a bit. So we boost it back up with+EV FC.
So, we do have to learn a couple of things about how our gear works, to know what it is going to do.
What flash do you have? In some cases, a Nikon solitary menu item of TTL will probably be TTL BL mode (for example, SB-700, SB-400, DSLR internal flash, and Nikon Commander mode).
