It clearly counts as terrorism if it's intentional damage to key international infrastructure by a non-state actor for political purposes.

(while the Houthis control much of Yemen, they are not its internationally recognized government)

That's leaning pretty hard on the technicality of who has the UN chair, with the detail that it's effectively us (NATO) vetoing their recognition.

Not that I'm cool with the cable cuts, but "rogue state" would be more fair imo.

I'm not entirely clear why state acts can't be called "terrorism"; examples would include the Libyan Lockerbie bombing, the French bombing of the Rainbow Warrior, and the Russian Salisbury poisoning incident.

That's fair as long as we're consistent, personally I always prick my ears at "terrorism" after seeing it politicized for most of my adult life.

I was more focused on the "terror" part. I would guess there's some line where a destructive act transitions from "attack" or "vandalism" to "terror".

The UK terror legislation (which is .. not great) defines it as "for a political purpose". Which is actually a reasonable distinction vs regular crime.