Is Alaska an enclave?

No. The article in the link mentions some informal uses of the terms enclave and exclave which are less strict, and by some of which Alaska may qualify, but in the purest sense:

an Enclave is a state, territory, or nation (or portion of such), surrounded entirely by a single different such entity.

An exclave is similarly surrounded but is bordered by more than one outside territory.

Alaska is an exclave as it borders (on land or by territorial waters) Canada, Russia and International waters and the U.S. territorial waters of Alaska and Washington do not touch.

https://en.m.wikipedia.org/wiki/Enclave_and_exclave

"An exclave is similarly surrounded but is bordered by more than one outside territory."

This part is not quite true. An exclave can be completely surrounded by one territory or by more than one territory. An example of an exclave surrounded by one territory is Oecusse which is completely surrounded by Indonesia.

https://en.wikipedia.org/wiki/Oecusse

Thank you I forgot that distinction. I should have worded it "an exclave can be surrounded by more than one..." rather than" is surrounded..." It is perfectly possible to be both an exclave and an enclave.

another key element is that an exclave must be a detached portion of a territory not connected directly to the main body. As such there is no such thing as an exclave nation...where an entire country is an exclave.

It can get pretty complicated particularly with the rules for treating sea borders. See the section on exclaves that are not enclaves (including Alaska because of EEZ territory)

And non sovereign semi-enclaves
(Alaska and Oecusse)

" https://en.m.wikipedia.org/wiki/List_of_enclaves_and_exclaves