AFAIK that's true. But if you know which Xiaomi/Aqara "friendly" router you want an end device to connect to (and if it's practical to do so) you can coax it to join via that router by keeping it in very close proximity (say within a foot or so, as long as incompatible routers are further away or temporarily unplugged) when the device is reset and hub is in 'add device' mode. Only the parent needs to be compatible (to handle the unique check in intervals of the Xiaomi end device); other non-parent routers won't cause issues.
If the mesh is stable, it will stay joined won't cause any issues. But partial power failures might require intervention should the parent router go offline while an incompatible router is within range-- it will try to rejoin via one of those and then drop if it gets aged out of the router's child table.
In practice, this hasn't been as troublesome as it sounds. I have a half dozen of the old Xiaomi round buttons (and a cube) which only stay connected via routers that can handle their extra long check-in intervals. My mesh consists of a few dozen Xiaomi-incompatible plugs and only two compatible routers (the C-3 is also compatible as a parent). So far in 5 years of using them I've had to rejoin them fewer than a dozen times, as a result of power outages (or has happened recently, when one of the two compatible routers was accidentally unplugged).
Rejoining is a snap; I used to take pains to unplug incompatible routers nearby but now just use proximity during the rejoin to get the device reconnected how I want it.
