Could be, as you speculate, that the Mesenchymals are more vulnerable in their walking/running state. But I have wondered if they have less Immune Blockade function making them more immunogenic / vulnerable to DCVax-L, with some other survival features that normally offset this weakness, perhaps this horrifying fast travel mode being one of them.
You may remember statements counter to that from the research; ie about Mesenchymal and Immune Blockade function. But since I don't remember any talk about that, I wonder if the other 3 subgroups have strong Immune Blockade/Checkpoint function in their arsenal, while Mesenchymal is successful without it.