You're right, that is crummy, and it is something that annoys me.
That being said, at least they include the clause. If this promise had been phrased as, "we won't link with Google products, but we might in the future", I would honestly feel a bit more charitable towards them. It wasn't.
DeepMind went out of its way to describe this not just as a temporary promise, but as a legal one.[0] In their words, "...data will never be connected to Google accounts or services, or used for any commercial purposes like advertising or insurance. Doing so would be completely impossible under our NHS contracts and the law".
That is a full step beyond the crappy TOS that most companies use, and statements like that were given to ease consumer concerns and make it easier for DeepMind to form deep relationships with hospitals.
One other thing to note outside of legal obligations or technicalities is that even if you don't think Google did anything different from the companies you're talking about, companies who do 180s on their TOS still lose trust. The trust loss is separate from the legality.
I don't know if you can go to Google and say, "legally, you shouldn't be allowed to link your product," I'm not a lawyer. I do know that we as consumers and engineers should instinctively distrust any future promises that Google makes about privacy.
If what they're doing is legal, I'm still going to call them out for being untrustworthy and dishonest. Their earlier statements are still lies.
[0]: https://twitter.com/MalcolmMoore/status/1062394654702927873