Why do you think the "silly" book writers would set discovery of the FO at 6 years prior to TFA instead of 25? Might it have something to do with trying to salvage whatever dignity they could for the OT heroes? Might it have something to do with not wanting to make it seem like Jedi Master Luke and General Leia knew about a threat like that for 20+ years and accomplished nothing in stopping it?
You might want to consider that perhaps some of those silly novel writers thought things through more than the actual ST writers did and didn't want to crap on the OT character legacies any more than what was already being done to them.
By rooting for the FO to become a known commodity in the Mando timeline, you're asking that it be likely that characters like Ahsoka, Bo-Katan, and Jedi-friendly Mandalorians find out about the FO 25 years before the events of TFA. But Luke and Leia couldn't do anything about it the entire two-and-a-half *decades*?
The outnumbered Rebellion (once Luke joined) wiped out 2 Death Stars and an entire Empire in less than 6 years, but now you want an entire New Republic, Jedi Master Luke, and General Leia to be incapable of stopping an upstart replacement they should've known about 20 years earlier? Why aren't you more concerned with preserving some shred of dignity for the OT crew's competency? If Mando is super satisfying right now while not touching on that stuff, why are you so eager for the show to take it on?
That alone will tell me whether or not to expect all the stuff from the alleged plot leak.
. We all know what was floating in that lab like the last pickle left in the jar. 
(a kindly attempt on F & F's part to help the team; but imagine after the fall of the Empire that there was a flood of folks trying to get to Exegol, 'coz at least Palps threw in dental benefits and the Dementor black robe if you joined up).
