A literal reading of the published rules would support your position, but it's worth noting that First fares with an LUTXV basis book into I or Z (Q/K also book into I, H books into D) so it's not surprising if the intended (or backend-coded) rule was to restrict to the same basis as an Economy ticket.

I don't know what your HUCA odds would be here; I have only booked certs into I (L) or lower.