Multiple updates for Safari are delivered over the course of six months. And note that the graph I've shown you plots the daily performance of browsers for the past year.

I agree with you that there was nothing wrong with Apple posting a page after a Safari release, comparing it with the Chrome version at the time, and then not updating the page until their next significant Safari release.

However, it is true that Safari gets feature updates much less frequently than Chrome or Firefox. Historically, there's the integer update in the fall (13 to 14 to 15) followed by the .1 update in the spring. Those are when features are added. The Safari updates that happen in-between are only bugfixes; some bugfixes may have performance implications but they don't include improvements to language support (HTML, CSS, JavaScript).

Safari 15.1 is already out but I think they only used that version number because it includes a significant UI change (to tabs), it's not an indication that they're moving to a faster release cadence for features.