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.
Agree on the first point, but is 'market share' - a metric comparing commercial sales revenue which by definition excludes open source software, really that relevant when critiquing a tagline for a FOSS tool implicitly targeting FOSS databases at a time when FOSS dominates?
If this wasn't an MIT licensed project without a single hint of a commercial offering I'd get it, but come on.