Apparently it's not good to require the router to start a new multicast address resolution on seeing unknown sources. It could be spelled out more but the text "This is particularly relevant for anycast and proxy addresses, where more than one node may be capable of responding" says that in some circumstances a lot of link local addresses might correspond to a single global address, and there could be a lot of traffic generated. So it's better to have the host opt-in to the prepopulating of the STALE entries.

"STALE allows the router to use the information it has already learned without requiring a new multicast address-resolution operation. The router can subsequently verify reachability using the normal Neighbour Discovery mechanisms."

edit: actually the RFC goes over this option as well and the reasoning there is slightly different than above (and maybe even the blog post) - see 8.9 at in https://datatracker.ietf.org/doc/rfc9131/ . Also notable that the RFC is from 2021.