Subspect you seeing realtime dynamics. Just checked Flight Status and both cabins are reporting full




It is not uncommon for a sold-out flight to bounce in and out of sold-out status as travel plans change

But you can standby in case space becomes available.

EF is reporting all inventory classes are zero'ed out (hence will nor be available for sale)

UPDATE
check flight stats on app and shows economy as AVAILABLE but ua.com reports FULL