I think Mueller is boxed in because the good guys can debunk any lies. Going mainstream makes any Mueller lies more conspicuous.
I think this is a desperation move. Dems are trying to scuttle an upcoming DECLAS by setting up a claim that POTUS is doing it to cover up or stop Muller's testimony, which would be damaging to POTUS (according to dems).
Dems have been trying to draw a premature POTUS response from day 1. Best thing is to let Mueller testify and nail him after the fact. I think we'll see delays and cancellations to draw the timeline out.
The other factor is the upcoming G20. DS needs to look like they can take out Trump in order to weaken his negotiating position. DS still betting on waiting until 2020 to vote Trump out and telling China and EU to wait it out with them.
This shit always goes down prior to a big POTUS trip. My 2 cents, anyway.