The second as is licensed by the first. It's not a question of modification, but rather completing a construction.
This is similar to an object being licensed by a verb at the head of a clause. The object does not modify the verb, but is a complement of it.
To be a bit more precise, englishgrammar explains:
The structure as…as is used to compare things that are of similar proportion.
In this case the first as acts as an adverb modifying the adjective or adverb that goes after it. The second as can act as a preposition or conjunction. If it is used as a preposition, it will be followed by a noun or pronoun. If ...
Welcome to EL&U, Joe!
The structure as + adjective/adverb + as is used to compare things that are similar in some way.
She was as cold as ice.
Your cat is as big as a dog.
He eats as quickly as I do (or ...as quickly as me.)
I am as hungry as you are.
Your example isn't perfect. Sloths aren't necessarily tired: they just move slowly! But of ...
Adverbs can modify verbs, adjectives, and other adverbs.
There are no other adverbs in the sentence, so that's ruled out.
The only real possibilities are the verb phrase "am with", and the adjective "best". The term "arguably" means that something may not be objectively true, but you're expressing a common opinion that it's true....
A dwindling reputation
Gradually diminishing in size, amount, or strength. Lexico
Declining; growing less
There is dwindling support for New Labour. Wiktionary
The decision in Ad Lib Club Ltd v Granville probably represents the
height of the court's willingness to protect a dwindling
reputation. M. Davison et al.; Australian Intellectual ...
Notice how easily "Her determination stronger than ever" can be transformed into
"Because her determination was stronger than ever." Absolute phrases strike me
as adverbial through and through. They modify verbs so far as I can tell.
Apposition would imply that you could replace the 'anchor' with the phrase that is in apposition to it and maintain the same meaning. This is not possible in the examples given.
The strings on the right could not replace those on the left while maintaining the same meaning.
He whispered to her. =/= An action that received an unwanted response.
He whispered ...
It seems to me that in a sentence containing "noun + appositive" both serve as a subject or object and either can be omitted. For a verb, similar conditions must be extant, i.e. either verb/lexical verb/verb phrase can be used. In "He whispered to her, whispered soothingly." this is not the case. -- "He struck out, expunged, the ...
Great question, and the short answer is yes, they modifier the modifiers while still acting as a single unit to modify the subject. By the way you worded your question, I can tell you understand the concept of compound modifiers, but I'll still include the information on them below in my answer - since others may have this same question while not possessing ...
The answer is that the first word in the compound adjective is sometimes a modifier, sometimes a complement, and sometimes changes the meaning completely. There is a brief discussion of this in the Cambridge Grammar of the English Language p1656-1657:
Noun + adjective compounds
The majority of compounds with an adjective as second component have a
noun as ...