Blockstream did not sign the Silbert Accord (a.k.a. the New York Agreement)
I don't see any current Bitcoin Core devs on the list of supporters either (the former lead developer Gavin Andresen did sign the agreement).
In fact, Bitcoin Core as an organisation would be effectively breaking its word if they did add SegWit2x logic to Bitcoin Core.
The whole premise of miners signaling for BIP91 was for SegWit2x, that's why it was not BIP148. It was a compromise. If Core holds onto this like you are saying, there will be a split. Also Gavin being the lead, his signature should've represented the team. Because the team changed, doesn't mean the agreement changes with it. I think it's healthy for bitcoin to grow up. I would think of it like a civil war. Businesses and countries have to sit on the side to see who will win. Bitcoin big block or Bitcoin w. SegWit. Competition is not only healthy, it is needed to prove which iteration will be more widely adopted.
If you support decentralization. Then you know there's no central entity and trying to keep it centralized is against its nature. This could be looked at in the future as an attack on bitcoin by one side or the other and the decentralized nature allowed both to try and see which version actually works better. Otherwise, it is centralized control where one side dictates to the other. I think this fork is by definition what decentralization was meant to be, not just nodes but decision making and the proper use of a fork. In my experience scaling systems, most of the anti-bcc arguments aren't true. A lot of this scaling stuff has been done 10x over by many centralized companies over the last 7-10 years. The amount of volume bitcoin usage on a system/network level is small compared to large orgs and highly inefficient. The rebel nature and passion behind it make it pretty powerful, but to unleash the power using configs from 2010 to prevent DDoS is a limiting factor. At this point, its preventing legitimate traffic and we know it.
From the blog post in your link... It's a choice.. Some people would rather leave onchain vs. adding "additional layers" while breaking up the original blocks.