The term is a nonsensical political designation.
Riddle me this... Why is the entire Island of Cuba "Terrorist" - but the CIA appendage, the MEK - a literal group of "Marxist" Terrorists, are not?
Ansar Allah - are part of the "Axis of Resistance" - to the Zionists' Genocide and Ethnic Cleansing, of all the Palestinian people(, and broader plans for * "Greater Israel)" ... A LITERAL WarCrime. Biden is attacking "the Houthis" (ie the Government of Yemen) for enforcing unilateral sanctions, no more illegal than the dozens of those, that the US and EU and NATO countries enact and try to enforce against any number of countries - including Yemen, Syria, Iran, Venezuela, etc..
* https://www.globalresearch.ca/greater-israel-the-zionist-plan-for-the-middle-east/5324815