It depends on what his assumptions are. Currently a 3080 produces $7.71/day in revenue[1]. If you extrapolate that out for a year you can break-even on your investment in less than a year, even if you pay scalped prices. However, this doesn't factor in difficulty increases, and depending on your forecasts it can vary from never breaking even to breaking even in less than two years.
Can confirm. I’m mining on 2x1080s and 2x2070 Supers, and definitely and easily turn a profit after electricity costs, however, it seems like profit is starting to drop rapidly to the point where I seriously question if it’s worth it. The GPUs run at 56c/120w avg. so it’s not too taxing, but it’s still putting wear on all the hardware, fans, etc.
Selling the cards (at above MSRP) now and putting that straight into BTC would probably turn a better profit long term, albeit, very much a gamble. The mining is definitely getting harder/less profitable and at some point soon (2021?) ETH is changing from PoW to PoS.
I'm just doing rough numbers here and I claim to know basically nothing about mining outside of a simple calculator off google. Hardware wise (more my knowledge base), I imagine the older hardware might have interesting economic characteristics - like if you could stock a fleet of company vehicles with really super great 20 year old Toyotas versus newer Toyotas. Problem is, finding that many 20 year old Toyotas of that reasonable quality, etc.
[1] https://whattomine.com/coins