We chat about videogames and videogame accessories.

I don't agree with classifying Mega Man Zero as a platform adventure. The fact that the areas are connected isn't significant to the overall game design, and if you took those connections away entirely it wouldn't be counted as one.

Basically, Zero's connected areas are more like if there was a door leading from Chill Penguin's stage to Flame Mammoth's that you could only use after clearing both of them. The game's progression is based on the missions, and you can return to an area after clearing a mission in it to find items you may have missed - and that's basically the same as every Mega Man game where you can return to Robot Master stages for upgrades, every Mega Man X game, and the other games in the Zero series. The only difference is that the areas are directly connected, but this is just done for the sake of connecting them, not because it's actually important or even relevant to the gameplay at all.

Going into your specific criteria, I would say it doesn't count as one because it does have discrete "levels" - the missions themselves are without question "levels" and are designed as such, and they're also the entirety of the game progression that you're required to do. The connections between them are an afterthought, never relevant to the gameplay.

If you had to actually explore the world in order to unlock new missions, that would push it over the line for me. But as it is, the game is "complete missions, then you can walk back to where they took place if you missed something or if you just feel like it, otherwise you can move on to the next mission".