It's relevant in that they're prolific databases, so there's a nontrivial chance that a user of one of them will:

- stumble across the tool

- read that it "works with their database"

- hopefully read the actual list of supported databases

- become disappointed to learn that it does not in fact work with their database

In the case that hopes are not met, the user actually downloads the application and discovers the hard way that it does not work with their database.

So market share does seem relevant to the frequency of disappointment though. Is it going to be a frequent enough occurrence to be worth spending any time doing anything about? I don't know.