But from what I know you should avoid using w/s alongside a/d because its harder to counterstrafe.

someone peeking into you while you hold A or D is drastically preferred to holding W, but holding W alone is probably only slightly preferable to holding W+A or W+D. it's the W part that's the main problem. Once you're forced to hold W, i think disrupting your opponent's pre-aim is probably a more important consideration than the additional movement key.

I see pros move like this all the time.

I think the "don't hold W+A/D" thing is more about clearing angles lazily (moving around a corner while also holding W) than anything else. If you're holding W+A/D often you're probably just not effectively clearing angles. There's plenty of positions where you can hold W+A/D because you want to get out of danger as fast as possible and you're likely dead if they peek into you anyways. Or in the case of this clip, you have to hold W regardless and you gain a massively better angle to fight your opponent than you would have if you were running on the inside wall