> IIRC every bubble he studied always mean reverted

This really depends on how we're defining these things. Let's call a stock-market bubble a period of elevated multiples. That can mean revert by prices decreasing while earnings stay constant or by prices staying constant and earnings rising. (Alternatively, both earnings and multiples can rise and fall.)

Yes, for equity prices in particular he talks about P/E ratios (among some other metrics like corporate profit margins), and so you're right, it would be possible for this to mean revert by prices holding stagnant and earnings catching up. However, as far as I can remember (primarily because a big emphasis of his was how unchecked bubbles can cause a lot of damage on the downside) all the historical bubbles he studied (something like 50) always crashed with a big price drop. Not 100% sure though, which is why I was curious if you had any contrary examples.