Fair enough; that is a decent piece of evidence, although I'd say so is the word of Nick Stadler. Basically I think that if the decision to remove Bilsy from the game hadn't occurred, he would be more likely to be fought in a different level. The name of that file suggests it might have been created solely to test the mechanics of the boss, and thus the tileset used would be whatever was at hand.

As for Bolly, that doesn't convince me, mostly because that would be a very loose remake, and in part also because the mechanical body of Bolly is very clearly controlled by a single Sparks eyeball, an enemy exclusively found in Tubelectric. Perhaps Bolly was selected by someone else to be a Medivo boss, but I'd say that minimally the artist's intention was to place him in Tubelectric.