This is normal and requires no explanation.
I know, I know, Peter Nielsen's answer has an explanation, with a citation and everything, and I voted it up for that. But even without Marvel's ‘sly humor’, this is exactly the kind of inaccuracy that you'd expect in a Broadway musical. If the producers of Rogers expect their audience to know Ant-Man, and they have him in later scenes, then that's all the excuse they need to put him in this one.
To demonstrate this, let me look at a recent real-world historical Broadway musical, Hamilton. Off the top of my head:
- Burr, Lafyette, and Laurens weren't in New York when Hamilton arrived and met Mulligan. In fact, Hamilton didn't meet any of those three until after he joined the army.
- Burr was not at the duel between Laurens and Lee. Lee's actual second was some guy you've probably never heard of.
- Neither Jefferson, nor Madison, nor Burr confronted Hamilton about the Reynolds affair. The conversation between Hamilton and three Democratic-Republicans did happen, but the three were James Monroe and two others you've probably never heard of.
- There are so many inaccuracies in the election of 1800 that it would take up too much space to list them all here. Suffice to say that it did not go down as shown.
- Also, the Presidential election of 1800 was not the cause of Burr's fatal challenge to Hamilton. The actual cause was a gubernatorial election in 1804, not depicted in the musical, where Burr lost to somebody you've probably never heard of.
And there are many others that I could think of if I checked but are not coming to the top of my head. I could also give examples from other historical musicals, such as 1776, Evita, and Six. But this is scifi, not history, so there's no need for a comprehensive list. The point is that Broadway (and the West End) do this sort of thing all the time.