Tanks doing more DPS helps the raid progress just like a DPS doing more DPS helps. Not to mention that better parses correlate positively with the tank’s ability to actually hold threat.
There’s a large element of player skill in parsing. Yes, group comp, overall raid kill speed, and RNG play large roles as well, but to say that player skill makes no difference on parsing is simply misguided.
Also, improving yourself on boss X doesn’t necessarily help on just that one boss. Often, those lessons can be applied to multiple bosses. If a lesson you learn on one boss can be applied to a boss you’re progressing on, that’s great news that will help your raid out.