He is right though, there ARE differences between the switches, so instead of saying it's "just preference", maybe you could indicate what exactly would cause the differences in taste. To put it kinda shortly:
Reds are the lightest switch. Therefore, your fingers will not be as tired if you adapt to them properly. Thus stamina will typically be the best with reds.
Blacks are pretty much a heavier version of reds. Logically, this seems like a bad thing, but the increased force also means the resisting force will push your fingers back up harder. There are some people who swear to blacks, so I would say if you already have very strong fingers so the increased force is not a big problem, blacks are probably better for you.
Blues have tactile feedback, meaning you can feel when you press the keys, but this also means the activation is not linear. Linear switches in theory would be better for a game such as osu!, but the tactile feedback can be valuable mentally so you know exactly when you're pressing the key rather than having to get used to the feeling of red keys. This is probably where the main "preference" aspect comes in, but I'd say that linear switches are generally better since our brains are very flexible and can get used to the activation of red switches.
Browns I believe are like blues but without the loud click, I'm not sure exactly how the activation mechanism works though so I can't say much about them.
In the end though, anyone is perfectly capable of becoming good with any of the keyboards. Having a certain type of switch that isn't optimal for you won't make you bad, you might just be a little worse than you could have been.