but shouldn't we also account for the hydrogen mr since the polymer as a whole at its ends will have repeating units that bond to hydrogen atoms?

That's a very good question, strictly speaking, we should account for the terminal hydrogen atoms (2 hydrogen at the end) However, it is negligible as compared to the entire polymer, so it can be neglected. Nevertheless, if you include the terminal hydrogens, it will still yield the same result when rounded to 3 s.f. as the deviation is extremely small. O levels convention didn't account of it so... But yeah, that's a very thoughtful question once again :)