Shutter Speed controls ambient light and Aperture controls flash.
It disturbs me when people say that, especially to the inexperienced. Whilst it has some truth in that shutter speed does not affect flash power when below max sync, it is misleading in that aperture does affect ambient and flash (if the flash is in M and you don't change the power level).
Don't be disturbed.
Play with all the variables until you find what works for you. Don't poopoo what the experts teach.
One has to have some basics to build on otherwise it is the blind leading the blind.
There really is only one rule:
make sure enough light hits the sensor to give you the pic you want.
3 basic variables: iso, f-stop, shutter speed.
If you don't have a foundation to build on then you cannot make any intelligent changes to how you use your tools i.e. your camera and lights/lighting.
So: Given your chosen iso then Shutter speed controls ambient and f-stop controls flash. See what happens when you try to control flash with your shutter speed. Not much effect!
Sure you can control ambient with f-stop, but it also controls flash.
The man is a beginner with flash. Has a long and fun road to learn. Give him some basics to start with instead of knocking what experts say.
Have a great day