1. There is, of course, no "ban on writing"
2. JMS hasn't suddenly "discovered" anything
In accordance with long-standing Hollywood protocol JMS cannot say too much publicly about projects-in-development with a given studio/network/what-have-you until after that entity has officially announced them. Hence his "coded" references like "2.35:1 hole" in the quotation above, referring to project as TWCBN ("That Which Cannot Be Named"), etc.
Similar restrictions on public discussion of negotiations and project development exist in plenty of other industries, too.
These things apply to
any comment by
any means, not specifically to writing. And they are less a "ban" than a conventional courtesy. It is the folks paying for the party who get to make the announcement and send out the invitations, after all, not the caterer - even if he is the one who will do most of the work.
None of this involves a "ban on writing", and WB certainly couldn't even
ask JMS not to write (or speak or send smoke signals) about subjects that do not involve projects he is working on
with them - like
The Amazing Spiderman. They know this and he knows it.
Why the sudden spurt of (not terribly long) comic book posts?
1. He's reached a natural "stopping place" in some of his other work and had time to peruse the comic book board - and was very annoyed at some of the silliness he saw there.
2. He's very busy with other writing but the comic book thread provoked him sufficiently that he responded anyway. It isn't like reading those posts and composing his replies would take
that much time.
As is so often the casee with JMS's activities, I think there is
less here than meets the eye.

Trying to tease cosmic significance out of the fact that he replied to a few posts on the comic book newsgroup strikes me as an even more bootless pursuit than most such exercises.
Regards,
Joe