That first benchmark is pretty dense, so it's not at all unreasonable to use KS.
On the other hand, if we force it to use KS for Z[x][y][z][t] down to Z[x][y], then classical down to Z[x], then Flint for Z[x] itself, it gets marginally faster, with 40.6s.
Clearly neither of these approaches is an optimal strategy, as the Maple timings show.