It's kind of an ill-defined term, and doesn't really mean that it was purchased whole. It could easily be a stripped lower + lpk + complete upper, or complete lower + complete upper. Either of those sidesteps the 10 day waiting period, and it's about the same price - you can get a lower+lpk with buffer tube and halfway-decent buttstock for something like $120, or you can buy a complete lower for about that same price.
Assuming you just buy a stripped lower and not an 80% lower, assembling requires minimal tooling - a couple roll pin punches, a hammer, some pliers, a hex wrench, and maybe another wrench. A vise grip makes it easier but isn't strictly speaking required. Takes like an hour or two even if you have no idea wtf you're doing.
That said, I agree it's probably more likely they bought it whole, or bought a lower+upper and just slapped it together (which takes 5 seconds and zero tools).