When I complained to Delta after returning I was told the travel websites buy seats in block and resell them, so that is why they could not give me a seat. Oh, and there were several empty seats on the flight.
Which just means that some people bought tickets and didn't turn up (and that there wasn't anyone on standby for that flight).
Airlines can't win: If they overbook flights, people complain about being kicked off; if they don't overbook flights, people complain about how there were empty seats even though they were told there were no tickets available.
If anything, this is technically wrong, as airlines don't sell "seats", they sell tickets. You don't have a guaranteed seat until your butt is in one and the door is closed.
But as the blog mentioned, perhaps the airlines have decided that overbooking is going to make them look bad during this event, so they are only selling inventory for available seats. Unusual, but it makes sense.
I suppose any travel agent could do that on-spec, but in normal operation travel websites are just brokers sitting on top of the same API the airlines use for their own sites.