I’d argue that they’re distinctive through ubiquity, not through design (e.g. there are other flags quite like the USA one that aren’t as widespread, so it’s clearly not the design itself that renders it distinctive. Similarly, the UK one has been incorporated into other flags, which helps its brand recognition) So they’re distinctive in spite of, not because of, their design.
Although IMO neither are bad flag designs.
And the “rules” are only guidelines - as I’ve pointed out in these threads before, Japanese prefecture and municipality flags break rule 3 all the time, and are awesome for it.