I bet it is their first and #1 priority, ain't like you or any of us get to see behind the scenes. Tell me, how exactly would you show off the progress of re-writing the MP from scratch? I imagine the goal is to make the MP nearly identical to the previous, from the player's perspective. How would you excite people with updates saying "hey, we now have player sync, look at them walk around!" or "these horde is synced to ALL the players on the server!" when it would look almost the exact same as MP in build 40 and brought nothing new to the table? The only way you'd really be able to show progress that looks noteworthy and ""exciting"" is showing people the chunks of code that make up the MP base, which isn't really that exciting when you don't understand code at all. I'll agree and say this update post is a little lack luster (looking at the sound images) but updates about MP aren't any different.
As for releasing it early, how is it that you don't see the writing on the wall when you say that? You got a ton of games from the past couple years that basically released earlier than they should've and players, justifiably, got upset. I guarantee that if they release it early like you want, all it would take is one bad experience caused by a bug (a server hitch while a horde chases you, leading to your demise for example) and the same players begging for MP would say "this game's shit" and they wouldn't want to play it again, especially if they were geared up and lost all of it because of a bug (on top of it, some might even say it was a cash grab from TIS, leading to more hate for doing the thing players asked them to do). Maybe you wouldn't think this way but a lot of players would, look at Fallout 76 and Cyberpunk 2077 as examples. I don't mean to sound dick-ish but how can you look at this and think it'd be different when history has clearly shown the opposite?