The actual announcement is here: https://github.com/omnisharp/omnisharp-vscode/issues/5276 It's not quite as dramatic as the OP is making it out to be.
These things almost never are, and the purists risk a purist death spiral. It reminds me of the closed source drivers debate. Someone has a proprietary component they want to incorporate into an open source system. The purists demand they open source that proprietary software if they want it integrated into the open source. The developer wants a compromise, but the purists won't allow it, so the developer is left to…
To me it seems more likely to be a culture clash - one team is used to developing closed source tools that have been highly prized by the company in the past, and another team has had great success writing open source tools that are widely adopted.
My guess is that the adapter code project is currently within the former team's remit, leading to this situation -- but that's all complete conjecture on my behalf.