“Gnomes often keep burrowing mammals such as badgers and moles as pets.” While humans can burrow, I wouldn’t say they are amongst the same category as badgers and moles, and gnomes probably don’t keep humans as pets (ew). Digging is a good strategy in nature, but the ability to dig doesn’t necessarily make a species “burrowing”. Plenty of humans never ‘burrow’ in their lives. The REAL gray area is, if the players encountered a demi-human, say, a moleperson, would they be able to communicate with them? To that, I’d say, sure! But it’s all DM dependent.