The uniforms worn by the Royal family are part of British history, going back to when they were expected to lead in battle and would actually have been wearing chainmail. Mock it, fine, but as someone sensibly said upthread, noone is going to think their new girlfriend Princess Anne is really a serving senior military officer.
This happens a lot with fake Paras/Marines/SAS/etc, some of whom use these fraudulent identities to collect money for veteran's charities which then ends up in their own pockets or inveigle their way into people's lives. I'm not suggesting that Fake Admiral is doing that, but we can't have people claiming service they never had or decorations they are not entitled to, whether they are just sad attention seekers or out to scam, because once we say " no harm done" it is basically giving the green light to pretend to be whatever you want, with all the benefits it might lead to.
ETA those who have actually done the hard yards to become Paras/Marines/SAS and everything else that is imitated have a perfect right not to like it.