I agree it isn't a big issue in the grand scheme of things, and I'm sure he can (and probably does) house rule it for casual games. However, I disagree about this being a "fun" book rather than a "competitive" one. I think it could be quite viable in a competitive environment (at least as far as Warlord books go in general). If this is his preferred spellbook, and he does wish to use it in official events, then the issue becomes a far bigger concern for him than it may be to us observers.
He makes some decent points, both mechanically and thematically, as to why it should receive such a change. The best counter argument I can think of is simply that errata sucks and should be avoided in my opinion unless absolutely necessary. The lack of a soldier subtype might not make a lot of sense, and might not be completely fair, but it's not absolutely necessary. It doesn't break the game any. Thus, my vote would be against errata. I'm fairly conservative on the use of errata though.
I do hope that your book sees support though in the future. It sounds like a very fun and creative strategy and having Arcane Wonders embrace it would be awesome.