I've been playing for a few hours on a map with Krastorio and Rampant. I had Rampant installed, and Migration enabled, since the very beginning.
Disclaimer: my evolution settings are pretty slow, and I did set the starter area size at 600% (that's the area around spawn point that's bug-free at the beginning), but I don't think that affects biter expansion. I did also set the enemy expansions cooldown down to 2-15 mins (min-max, default is 5-60 iirc) .

See map screenshot here
https://ibb.co/Xr9H667N
I highlighted the bug nests. As you can see, there is a big blob of the just north-west of my base that is slowly expanding, mostly towards me, attacking often as expected. Then there are other small nests around the world, but they don't really seem to expand.
I was expecting most of the world to be overrun in their bases by now, since
- I've been playing for a while
- I set the expansion cool down pretty low
- I've activated migration mode (which is warned to be Survival Mode and really hard, but that hasn't really been my experience).

Do biter nests only expand when exposed to pollution?

Setting 2-15 minutes means that at the beginning of the game the settlement will be once every 15 minutes. This is not so much with a large starting zone. In addition, the standard settlement goes all over the map and you do not see most of it.

If the nests are very far away, even the "migration mode" takes a long time to populate the map. Since chunks with nests are not created due to the distance from the player, the AI ​​thinks that there are few nests and it generates few settlement squads, but gradually the settlement will accelerate. But rest assured, eventually the map will be covered in nests. It just takes more time.