Is fleet here an adj. or n.?

[v0031]

 Battle of the Philippine Sea - June 1944

The last great carrier battle of WWII, the Battle of the Philippine Sea happened as U.S. forces advanced across the Pacific. A Japanese force including five large fleet carriers and four light carriers, plus some land-based aircraft, fought seven U.S. fleet carriers and eight light carriers.
The U.S. enjoyed not only numerical superiority but also vastly better aircraft. The new Grumman F6F Hellcats outclassed the old Japanese Zeroes. This disparity led to the action being nicknamed "the Great Marianas Turkey Shoot," with about four times as many Japanese planes downed as American.

[Wells]

noun

[Citizen (The) Doctor]

noun

Isn't this technically an adjective in this case as a fleet carrier is a possible descriptor for the largest carriers?

[True Federalist (진정한 연방 주의자)]

noun

Isn't this technically an adjective in this case as a fleet carrier is a possible descriptor for the largest carriers?

It's a noun adjunct, which is a noun modifying another noun.  It's not an adjective because you wouldn't use 'fleet' ns this sense as an independent adjective.  That is, you could say "The USS Enterprise was a fleet carrier,", but you couldn't say "The USS Enterprise is fleet." (Or rather if you did, it would be understood as the adjective 'fleet' meaning fast rather than the noun 'fleet' meaning an assemblage of ships.