According to the wiki, it's not being adjacent to a settlement that has opened the caverns that activates underground plant growth, but rather it's being located above a cavern that has access to the surface. Subtle difference in semantics, but the upshot is that, in all probability,
1) At least one of the caverns under your embark extends beyond your map.
2) On one of these off-map grid squares, there is a cave that connects that cavern to the open air.
This makes the game treat the cavern as "breached," hence, you've got floor fungus.
Your dwarves knowing about magma heat is probably triggered the same way: It's quite possible that the cavern (assumedly the 1st one) has direct access to a volcano or magma pool, and if the cavern "knows" it has that, then so do your dwarves living upstairs. This could be good news, meaning that you don't have to dig far at all to get free heat . . . or, it could be bad news, if the volcano is on a grid square that's off the edge of your map. You might wind up with a Secretive Armorer demanding a magma forge, and you have absolutely no way to give him one. (I don't know if moody dwarves can insist on a magma 'whatever' when you've got a perfectly good coal-fired 'whatever' sitting there . . . but given the levels of dwarven stupidity I've seen, I wouldn't be surprised.)