I have no way of knowing how much these people paid, if anything, to the dev's Patreon or Liberapay or Donate to PayPal. What they did do was spend their time reporting serious issues for zero payment. Bugs are important to fix, some bugs can be critically important to fix. If the dev chooses to fix these bugs, it is the dev that will receive all credit for doing so. The person who made the effort (albeit minor in comparison) of reporting the issue will not be credited or acknowledged in any way.

Also, I think a vulnerability that allows unprivileged users to gain root on a system on which Calibre is installed is worth fussing over.