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.
The UK terror legislation (which is .. not great) defines it as "for a political purpose". Which is actually a reasonable distinction vs regular crime.
(while the Houthis control much of Yemen, they are not its internationally recognized government)