Am I misunderstanding what BTF is and the role it plays in BPF? Or
maybe has libbpf development moved so far toward CO-RE that non-CO-RE
development gets similar or the same error messages that just aren't
as clear for it?

Hmm, no, CO-RE is the specific feature that does relocations of struct
fields based on member names. This relies on BTF, but it's not the only

CO-RE is more than only field offset relocations, btw, you can detect
type and field existence, get type size, use relocatable enums
(internal kernel enums can get renumbered, so this feature allows to
accommodate that), etc.

thing BTF is used for. The map definition is another, as you discovered,
and there are some program types that cannot work without BTF
information at all. Also, things like bpftool being able to print out
the struct layout of map values is using BTF. So you're certainly right
that the BPF ecosystem in general is moving towards using BTF in more
and more places. And I guess you're also right that this leads to some
cryptic error messages sometimes... :)

Thanks for your reply, Toke. I don't think I added much value here :)