DoS is different from DDoS. A DoS attack is any kind of attack in which the attacker seeks not to gain access to a system, but to simply deny others access. This can include sending large payloads, maintaining idle connections, causing crashes, logging out other users etc.
A DDoS is a particular kind of DoS attack - one in which the attack is conducted using a distributed set of machines, usually helping to (a) hide the attack from monitoring tools and (b) make it hard to block using IPs. Obviously this was not a DDoS, but could have looked like a plain DoS.
The definition of a DoS attack depends on the system being attacked, there is no hard and fast limit to how much traffic a system is supposed to handle. For a badly written system like the one described here, the actions of the scraper were indistinguishable from the point of view of the library from the actions of a malicious actor. Hopefully, since the intent was not malicious in this case, thing may have been cleared up - though of course it's also possible that, unfortunately, they haven't.