The "unpaid taxes" in this case are not the pilots dodging things, but the fact that because the state deems BRK/Storm (or with any luck, ultimately Ryanair) to be the employer, these organisations are not providing proper employee benefits such as sick, pay, employers' contributions and it is this they are after.
In my time as a "service provider" I had maybe 5 interactions in total with my "agency" and everything else was entirely through Ryanair. This included changes where Ryanair dealt with and actioned moves and the agency paperwork caught up months later. HMRC are also after Brookfield for similar reasons.
Under UK law, it is BRK who would be liable for any financial penalties were the pilots found to be employees, and anecdotally Brookfield is being wound down, with contracts not renewed but transferred to Storm or Ryanair terms.
I don't know what the situation is in Germany.