On the one hand its Gargrons right to allow whatever services he wants to access his API. This is no different than blocking a fediverse server. That said I think its unfair to call this a DDOS or even a misuse of the API.. In fact I'd say this is the very purpose of an activity pub endpoint, so anyone can access the data and interoperate with the server as they see fit.
@freemo @mkljczk @xorowl @jimpjorps It's DDoS when their software is coded in such a way that many IPs hit the same endpoints over and over in frequent fashion when they could simply cache the results