I think it kind of depends. I've seen multiple definitions, and one of the fusion definitions is that "if the characters are there as if they've always been there..."

That definition doesn't work for me.

I consider it crossover if both worlds are canonically correct and the characters retain their relationship to their own world.

I consider it fusion if one world is changed by the addition of the other, thus "fused" with the other one. If characters take on new backstories and current work/identities, etc. or merge with another character (actor fusion), then that's a fusion.

As yours retain both source canon and simply let them meet/interact, that would make them crossover from what I understand. The definition of fusion I reject would call it a fusion, which is why I reject it. It's too vague.