What it often means is that the advertiser is tendering for a contract which has certain minimum crewing standards and numbers. They will use the responses to “Expression of Interest” as if they already have these people on the payroll.
Sometimes they won’t even have the required aircraft, but may (only maybe) have potential leases lined up if they get the job.
You will sometimes see similar ads pop up around the same time from more than one operator.
SOP in the world of BS aviation marketing.
If I was in need of a job I would have no problem expressing interest to all the advertisers. You could get lucky with one of them. As long as the operator identifies itself. Post Office boxes with no Company name are strictly a no-no, as that suggests the operator is dodgy or the advertiser is a wannabe recruiting agent looking to make a quick buck for doing SFA.