Not all manipulation is gas-lighting. Gas-lighting is when you intentionally create a scenario to make another person doubt their competence as a distraction from your own nefarious actions.
If the boss had intentionally given the subordinate a task that was harder than it seemed in order to make the subordinate feel inferior, causing the subordinate to feel at fault for the lateness of a larger project and therefore not report the boss' bad decisions up the chain of command, that would be gas-lighting. But just telling them they are at fault is not gas-lighting.
To clarify, I don't mean to say the boss did something ok, just that I think the term gas-lighting as a term is very useful to describe a particular phenomenon in various relationships, but is less useful if it gets broadened. I think this is something different, not necessarily better or worse.
That was the point of the meeting, to find out more. Since nobody volunteered any additional information contradicting their view, the lead manager had to go with what it looked like: a project was late for no good reason.
Then why even have a discussion about something you don't know.
It looks a straight up case of cya, followed by manipulating a junior employee into believing it was their mistake all along- a.k.a gaslighting.