The credits level has gameplay. Sure you can't game over, but you can play it much faster than if you just set the mouse down and let it run. Also, the credits level has its own results screen just like any other level, because it is a level.

Also, the fact that you can be forced to play a bonus level (gameplay) after the credits should prove that the game isn't truly over yet. The bonus levels could probably be strategically avoided anyway by not catching treasures.

Besides, the game does not say "X Difficulty Cleared" until the results screen, so it is reasonable to assume that you have not cleared the game until you reach that screen. You won't even get a registered high score if you quit before the results screen, because you didn't complete the game.

This is not the only game I've seen where the credits are put inside of their own level. Just because the credits are put inside of a level doesn't mean it isn't still a level.

Personally I think time for speedruns should be called at the results screen, rather than beating the 8-4 boss.

The credits level is a legit part of the game. It can be speedrun, and it applies to your high score.

Also, it's very possible (and in my experience, common) to enter bonus levels even after the credits level is over.