In worst case scenario you can’t still get flaky test, right? Single thread runtime that will allow the queries to interleave sometimes and sometimes work correctly - talking about variant without "FOR UPDATE".
Right, that's a real concern with naive concurrent tests - you're at the mercy of timing and the test becomes flaky. That's exactly what the synchronization barrier solves: it forces both transactions to reach the critical point before either proceeds, so the race condition is guaranteed to occur on every run. No flakiness.