You're correct that expressions like "once-bravado-filled" do exhibit some characteristics that are often associated with text generated by language models like GPT-3.5. Such models are trained on a vast corpus of text and can generate creative and descriptive language. Phrases that combine words in interesting and unique ways, like the one you mentioned, can be a hallmark of text generated by these models.
However, it's essential to note that while certain phrases or styles of writing might be indicative of text generated by a language model, they are not definitive proof of authorship.