"It is hot."
In a sentence without subject (like English "It's hot." "Estas varme") there's no noun for an adjective to describe. So we use an adverb to describe the verb "estas".
There's the famous sentence "Resti kun leono estas danĝera/danĝere."
In this sentence both are correct because you can interpret the clause "resti kun leono" as subject of the sentence, that can be described by an adjective, which would lead to "danĝera". You can also interpret the sentence without definite subject which then would lead to "danĝere".
In the simple case of "Estas varme." the adverbal form is the correct one.
You're undoubtedly right about "Estas varme", but according to the Akademio de Esperanto, PAG and PMEG, only "Resti kun leono estas danĝere" (as the sentence is written in the Fundamento) is correct.