I don't think you understand their business. Contingency recruiters' fees can reach 30% or more of an engineer's first year of salary, which could work out to $50k or more per engineer. You can bet they will be looking closely at each candidate's solutions, plus automated code plagiarism detection is pretty easy.
It also depends on the type of problem. If you just need one weird trick to pass the level (e.x. use this SQL injection) then that's easy to game. If you have to write a bunch of distributed systems code (e.x. Stripe's last CTF) that's a lot harder to cheat.
It also depends on the type of problem. If you just need one weird trick to pass the level (e.x. use this SQL injection) then that's easy to game. If you have to write a bunch of distributed systems code (e.x. Stripe's last CTF) that's a lot harder to cheat.