Negotiations between Rome the Central powers did continue even after the London Treaty was almost secured, but Vienna’s reluctance lead to their failure. Despite Germany supporting Italy’s demands, Austria initially completely refused any concession, going by the rule that any territorial grant on a national basis would lead to the collapse of the Empire. Demands included Trentino and an extension of the eastern Italian border on the Isonzo river, along with an autonomous status for Trieste. As time went on, Vienna gradually came to reluctantly accept these demands, but by then Italy had already signed the London Treaty.

So yeah, not really an absurd scenario if Austria had more hindsight.