radiation can interfere with electronics.
again just some kind of simple coating.
most types of radation can be stopped by a thin layer of tin foil.
That's not true. Check out this Wicki article on the effect of ionizing radiation on electronics, and the extent some computers are "rad hardened" by fairly extreme methods. Some hardened systems have three computers vote against each other, to mitigate radiation damage:
WWW LinkRemember there are some extremely harsh environments in space, nothing like what we have on Earth or anywhere in the Solar System. When the game talks about radiation planets, for all we know they might be referring to planets being bathed by radiation from nearby supernovas, or being swept by axial radiation beams from neutron stars or black holes. The Yor were designed by the Iconians to operate as servants in whatever environment the Iconians can tolerate. Since the Iconians need extreme colonization tech to live on high radiation worlds, it's reasonable to assume that the Yor might need it also.
On the other hand, appeals to logic don't go that far in this game.

It doesn't make much sense that the Iconians built the Yor and were defeated by them, but somehow the Iconians retain a natural ability to colonize Toxic and Aquatic worlds, but the Yor never managed to steal that tech from the Iconians? Well, maybe they developed that tech after they had to flee their homeworld.
It basically comes down to game balance more than logic. Some of the races that aren't as strong in the early phases of the game, get a few more planet types they can colonize.
The Yor already have one of the best super abilities, one of the colonization techs, and in general are massively powerful. They don't need any more advantages.
Well, their economy is a little shaky in the latest TA betas, and they're light on influence abilities. I don't think getting one extra planet type to colonize makes them too powerful.