You know, it may be that WatchOS 5.2 is delayed due to regulatory approval for the ECG in the UK and elsewhere not being quite ready. It is such an otherwise inconsequential release, I can't think of anything else that would hold it up. I've seen almost zero problems reported online about the beta (and I'm running it and its been flawless for me).
See! I'm coming around to your theory. There will still be a significant announcement, likely via a local event (where's Tim Apple today?) and a press release.