The fact that Abu Lahab is an epithet of an actual person, not a metaphor.
As a non-Muslim Arab, how much do you trust Islamic theology to be an accurate representation of historical fact? Do you think the Sirah is broadly accurate, or largely invented for theological reasons (or 'other')?
If you look at tafsir from the likes of Tabari, it is clear that mufassir have absolutely no idea how to interpret many passages of the Quran and are merely guessing. Even to things that should be important like who the Sabians were they have no idea. Other passages they are plain wrong such as associating al-fil with Abraha, or not understanding why Abraham's wife laughed.
On the other hand, minutiae of the Prophets life are recorded with stunning accuracy such as women lobbing sticks into people's paths.
To me, Abu Lahab seems far more likely to be a metaphor as part of a parable than an actual person, and the story seems likely to be made up like many other things clearly are.
He is called the "man of the fish" instead of being referred to with his real name. Does that mean we should just dismiss any exegetical commentary that points out that this is one of the names of the Islamic Prophet Yunus?
This is an intertextual reference though which makes it clear. The Quran is full of them, although the mufassir didn't always realise as their knowledge of pre-Islamic theology diminished over time.