The mask definitely sets a limit alhough it isn't as hard as a LCD etc. with acutal fixed pixels, because a dot in the triad can show info from more than one logical pixel in the incoming video signal. Beam from one line can partially light up a physical phosphor dot and the next line can light up that same one and you get to see info that would otherwise have gotten fully lost instead of partially. In a sense it gives a level of subpixel rendering for "free", it is one of the reasons why going past mask limits still gives something that isn't completely ass to look at, at least when the limits are not gone past by a huge amount 🤣. Horizontally though, things happening between the triads, for one color, is lost but you get info from a different pixel on a different color between, so there's another temporal layer on the mix which helps to enhance legibility by a slight amount. The same way can be used on LCDs etc. also if one really wanted to.