They are allowed to set caps, I've seen them down to around €5000 per transaction so it's not literally all transfers. Still free to transfer above, just takes more than a few seconds to do so, presumably for fraud/theft protection. Some banks will make that a daily limit, or configurable, there's some flexibility but they're mandated to justify all limits with no arbitrary discrimination against instant transfers.

In practice, obviously most personal payments are well under that margin, so within-EU bank transfers are defacto instant everywhere. Most banking apps I've seen don't even show an 'instant/slow' option any more, it's just all instant.