HR being equal does not mean that all related things are equal, especially when it comes to fatigue.
Maybe if you attempt to define "tired" more precisely, that would lend some clues. My initial thought would be that the swings localize the work to certain muscle groups a bit more than the global work of running.
Also, is your HR the same for the whole period of time? Like, if you compared two sessions, one session with 5 sets of swings with 2 min rest between each, and the other session running intervals with enough rest to be the same total time as the first session, would your average HR for the entire session be the same? (Not that this would answer it, but would be another data point to consider.)
I would have to agree here. I would imagine swings generate more muscle fatigue in different areas than running intervals would.
If I run my heart rates up and my cardiovascular system is putting in work same as swings, but during swings my grip is working and my back is braced and my hips are firing harder. It's a different kind of tired at the end.