if the algorithm is open source, bad actors can see it as well. it'll become remarkably easy to spam and game the system. this could drive users away, decrease revenue and leave Mr. Musk holding a $40B hot potato
It seems like the focus needs to be on what can fight back misinformation/spam better given the context: more speech or moderation. The need for human moderation will likely still exist, but if Musk wants to accelerate the process as you say, Bluesky seems like a good way to start.
If Twitter is making choices that increase their bottom line at the expense of community, then absolutely, more transparency and open source would help with that.