Writing documentation and fixing bugs is in fact not the bar for a senior software engineer. I don't disagree with the promotion committee on that front. IIRC, Senior Software Engineer is a terminal level. You aren't expected to advance any more once you reach that level. Some do, but you can stay a Senior Engineer for the rest of your career and that's fine. So certainly "can fix bugs and write docs and tests" isn't…
The lack of metrics is a problem. But the bigger problem is the interesting non-maintence-mode projects went from 0 to somewhere, or from 7 to 8 and a bit features' and that's a lot 'cooler'/'bottom-line demonstrable' than 'I made the thing break less often and less breakable and more understandable'.
It also says something that Google isn't measuring itself very well, although this is one anecdotal data point, I'm sure it's not the only one with the 'defrag' reference, relocation/churn etc. etc.
I also take issue with your documentation reference. The existing lack of documentation was probably caused by other Senior Software Engineers?