Well I was under the impression that the whole thing re Deep Ones was a cover to tell the patsy Maester to kill someone associated with Viserys from the anti-magic maesters. But given the Lantern bearer maybe it is someone trying to do anti-deep ones shenanigans
I figure the idea of it being anti-deep ones was a sop to the Lantern bearers. If we can cast suspicion on what other things the Maesters lied about to them all the better.