I'm not sure it's quite that easy...
There’s some rather shonky logic in the commentary here, IMHO.
First, I'd have thought that the starting point is to work out who is the controller of the processing which takes place on each key server, on what basis the data are being processed, what rights apply to the data subjects, whether any exceptions apply, whether any exemptions apply, and so on. Without this, it's all a bit nebulous.
Similarly, the reference to "implied consent" sounds like a red herring, since consent requires a "clear affirmative action" by the data subject — it is either "consent" or it is not — and, in any case, (a) consent can be withdrawn at any time (Article 7(3) GDPR), and (b) the right to erasure, under Article 17(1)(b) expressly applies to processing done on the basis of consent, where that consent is withdrawn. So, even if "implied consent" is a thing, you can't argue "implied consent" as the basis of continued processing, in the face of an objection / request for withdrawal of consent.
Lastly, I’m not sure where the concept that “the right of erasure only applies where it is practical” comes from. The right may not apply where the request is manifestly unfounded or excessive (Article 12(5)), but that’s hardly the same as whether the deletion is “practical”.
I suspect we simply have here a situation in which those designing and operating the key servers did so — perhaps entirely reasonably, at the time — without considering this kind of issue.