Not an autoantonym; that word only has one meaning.

Its problem is that it appears to mean the opposite of what it means, not that it has two contradictory meanings.

“The mighty sequoia, protected by thick bark and with its foliage typically high above the flames, was once considered nearly inflammable.” — Brian Melley, Anchorage Daily News, 23 July 2022

That’s just misuse of the word.

Sure but by now every rational and reasonable reader must understand that whenever they come across the word "inflammable", the author might have misused it and they need to search for additional signals to identify whether it means "inflammable" or "uninflammable". This is not different than how the rational and reasonable reader treats the use of an autoantonym.

There's a difference between misusing a word, and a word that has two accepted meanings.

Brian Melley needs to be acquainted with the word nonflammable, which is the one he really wanted to use.

There's no need to use a single word; that sentence is trying to sound impressive, not concise.

Try "The mighty sequoia, protected by thick bark and with its foliage typically high above the flames, was once considered nearly impervious to fire."