There is no incompatibility between MIT and GPL (although there is between Apache and GPL, and parts of Squeak are still under ASL 2.0).
There is no incompatibility between Apache and GPL3, but that's secondary.
But GPL is viral (and LGPL too if you rip out code and not just link to it) so it infects other code used together with the GPLed code. We do not want Squeak to become GPL licensed so we need to be careful what code to let in.
And to be clear about this once for all, I completely agree with the above assessment. Squeak people do need to be careful, even a bit paranoid if you want. But implying that you cannot look at GNU Smalltalk for fear that you'll glance at its code and be tainted by it, goes way beyond paranoia.
Paolo