Well, technically Collins said that it was an action that the FHFA could take under the Incidental Powers of HERA.
Right.
A different question presented may be better: Can two federal agencies (here the US Treasury and the FHFA) act together to Nationalize two private corporations?
That depends on if you consider receivership to be nationalization. If so, I would answer your question with a "yes". If not, "no".
What they evidently can do is a de facto nationalization where the government gets all the control and all the money but keeps the shareholders around in name only.