If you take a look at the videos posted above, slings could do serious damage to armor! Taking a hit in the head even with a helm is not a good idea!
1. I said I would add it to the stack of things to watch.
2. You cut my quote around the fact that I said, in effect,
'yes, I'm sure it had something to do with it, but... {other more compelling reasons}'
Regardless, yes, slings come in with maces and warhammers as weapons that 1) are hampered by armor, but 2) still are effective against armored opponents (and not by going around the armor). If it doesn't penetrate, it'll still really ring someone's bell and damage the armor. That gives certain contexts to when they ended up being used. It's always a subtle dance between how well something works and how well alternatives would do.
Maces were common in the classic era, and then saw a resurgence in the late medieval/renaissance. Much of the era in-between, the common tactic was instead to use sword or spear and try to get around the shield and mail of the time (axes being the dual-use 'if you can't evade, this still telegraphs a lot of force through' weapon of choice). Once plates got more common, the tactics switched to* punching through with narrow spikes/half-swording, or going back to ringing the other guy's bell. If stronger and stronger bows, crossbows, and firearms hadn't come along, there might have been a resurgence in slinging (although again, highly skilled lightly packed footmen that work better for skirmish/disruption than primary takedown...).
*along with pikes to stop horses, falchions vs. lightly armored troops, and all the other exceptions I know someone will point out if I don't mention them.
Also, just want to clarify he is discussing the 5e24 rules as he specifically comments on weapon mastery properties. He likes most of them for flavor but would change a few (he doesn't think "graze" makes sense for the greatsword and suggests cleave maybe).
There's undoubtedly many of them that are iffy at best. Graze might really be better for maces/hammers/slings, representing the above effect.
PS - he is clearly trained in many weapons, but bows are his specialty. His knowledge and skill regarding archery is impressive.
That's good. I haven't seen as many of those around. Nice that he's a millennial or younger, as I think most of the people I tend to think of in the medieval arms Youtuber crowd are us fogeys. Also nice that he looks more like a gymnast than a power lifter. For the past decade or two (probably since 4e put dex as the attribute for to-hit and damage) , realism discussion around bows seem to focus excessively on gamers rediscovering that, yes, archery requires a lot of strength and every archer shouldn't look like Orlando Bloom/Errol Flynn. I'm sure he can bring some nuance to the discussion in the 'it takes everything' vein.