IMO, it's a scratch/gouge possibly caused by something falling against it as the OP believes, or the (IMO more likely) the bike falling against something. I wouldn't consider it especially severe, especially since it's a way's back on the top tube.

In normal riding the top tube is a compression member so there's more forgiveness than if this were up front on the down tube. The only issue is that it may accelerate local fatigue, though it's anybody's guess by how much.

If it were mine, I'd blend it in a bit with a file and/or emery then clear coat the area. Then I'd ride it as if nothing were wrong, keeping a weather eye for any crack formation. I'm not taking about a close watch, just a glance now and then. Odds are it could last until for years, outlasting your attention span.