From what I can see they actually started pushing the steam update to stable users before the epic tweet. And has been on the beta channel for weeks.

So I wonder if it's the other way round, Epic rushed out the boast because they knew the window that it would be even remotely correct to most users was closing.

The CEF update was baking in the public beta channel for around the last 6 weeks.

Epic could have benchmarked their beta client against the Steam beta client which would have shown this.

If I had a performance improvement ready for release, and my competitor started bragging about being faster, I'd for sure jump at the opportunity to release it sooner.