They may simply have missed it or failed to realize the relationship before so blindly saying "it's fine I will just ignore that they skipped over my own work" when it is in fact related doesn't seem like the right approach.
If it really is unrelated I doubt anyone feels the need to point to their own research.
In practice, reviewers did sometimes request citations of their own works. As an editor you could reject the whole review report, but you were on deadlines and quotas and you typically had a hard enough time finding reviewers anyway. In my experience, most of the practices that researchers gripe about in MDPI are results of onerous working conditions rather than any kind of laxity in the culture; everyone was regularly encouraged to be serious about scientific integrity and seek out the best reviewers, and practically all of the people I worked with were quite intelligent and concerned about doing a good job, but it was also an unspoken expectation to work after hours, Saturdays, to always be sending an email, etc; exhausted people make mistakes.
Also, I think people are overestimating how much reviewers care who the authors of the papers they are reviewing are, at least in my field.
For one thing, the papers are often written by a student, so writing style isn’t a great giveaway. Also, the advisor probably appreciates a thoughtful rejection of the crappy student paper. The alternative is getting it accepted, followed by a mad dash by the advisor to copy edit it and add experiments so the work isn’t embarrassing. (Tenure clocks are the main counter example of this, but junior professors aren’t exactly in a position to bully the reviewers!)
People could try to block work that would scoop their ongoing paper, but I don’t really see that happening in practice in my field. Double blind doesn’t help with that, of course.
However, it would pretty rapidly tank that reviewer’s reputation, since the other reviewers would see the bad behavior, and they know each others’ identities.