Both metrics have their uses. Personally I'm not all that interested in playing the latest hit AAA games. I am much more likely to play indie games or older games, which probably means I play more titles but the titles I play tend not to have much market share (individually or even in aggregate). You could say I'm an outlier but I feel like a lot of Linux nerds probably have similar gaming habits.

So for me it's better than 90% of all games are playable on Linux, than if a handful of games accounting for 90% of market share were playable.

Ignoring the Steam Deck, most Linux gamers are probably going to be older so that they have the funds and/or technical interest to setup a Linux machine. Which is going to remove the younger demographic which likely plays a different genre and has more available total free time for gaming.

As a young teen with nothing to do, I probably had some days in Summer where I clocked in 16 hours of gaming.

yes, but not marketshare played on linux, but rather, how much of the marketshare that's currently played on windows would be playable on linux.