When you roll out an auto-updates mechanism you're saying to the people who enable it "you can trust us to do the right thing with your project while you are elsewhere -- this is a risk but it's one we manage for your benefit".
If you roll out a change for purely political/commercial reasons that are ultimately not your end user's concern -- we're not a party to that lawsuit -- then you're undermining the trust in that mechanism entirely.
It was a stupid, arrogant, underhanded thing to do.
I don't know off-hand what the rule is for plugin updates, actually; I'd have to look it up.
As far as WordPress itself is concerned, the updater definitely does not auto-push updates to major WP versions by default [0], and they continue to patch older versions for a long time.
But at any rate, whether the plugin updates respect SEMVER or not, Matt/WP.org pushed this bullshit out as the most minor of minor version number changes over the previous ACF version: 6.3.6.2.
https://wordpress.org/plugins/advanced-custom-fields/advance... (scroll down to the bottom and you can download the previous version to diff it)
So as far as the poor benighted plugin updater is concerned, it's just a change to the display name, which is inconsequential.
[0] WP Engine do, ironically, on a pretty short timescale!