Makes sense. The article calls it "unwarranted gatekeeping," but they wouldn't say that if they knew how Android internals work.
Looking at the video demo for Circle to Search, it's very likely they built this on top of ActionsServices, an Android component that enables extra interactions on top of the Recents switcher. This is already what's being used to do things like OCR in the Recents switcher.
Other non-Google ROMs use ActionsServices too, but their implementations vary, and they can't just "tack on" something as complicated as this onto any vender implementation of ActionServices and expect it to work. They might not even have a vendor rollout plan for this thing yet, for all we know it was rushed out the door.
Google has had a tight partnership with Samsung since the Pixel 6 came out, which is why it doesn't surprise me that Samsung will be getting this feature first. Google can essentially boss Samsung around for little system things like this.
The "for a long time" comment was probably due to Android 15 already being mostly final at this point. I wouldn't be surprised if there were core changes to Android to allow more pluggable customizations to the Recents switcher in Android 16. That might enable Google to offer this feature to other vendors more cleanly (assuming the feature survives that long, which is doubtful).