From what I understand this does not prevent use of custom ROMs, it just means ROMs built before it was done will not work anymore. I assume they can re-package old versions to work with the new configuration, I am not entirely sure though. There are discussions elsewhere in this thread with more informed people.

it just means ROMs built before it was done will not work anymore.

From the article:

Any subsequent attempt to install older firmware results in a permanent "hard brick" - the device becomes unusable

This implies that not only does an older custom ROM not work, but neither does attempting to recover by installing a newer ROM.