POIs for normal artillery rounds reach "up" the same amount of distance as their radius; that means they may have an effective range of 0" to the aerospace unit as far as determining range is concerned, but an AOE can't hit anything on the flight path, because it's at a higher altitude. After all, the range to a VTOL (for example) is unaffected by its elevation, that just happens to have nothing at all to do with where the explosion goes off. That part is easy.
Moreover, "it stands to reason" means exactly what I used it to mean: it makes sense. You're lasing a target. In Total Warfare (oh, how I hate using Total Warfare to justify rules, but here it makes sense), Homing rounds are fired at mapsheets, and I'm fairly sure (but curiously can't find it explained explicitly in the text, for reasons I'm sure have to do with how much of an edge case this is) that a fighter that crossed through that mapsheet is a valid target.
And much more importantly: it doesn't break anything, in the slightest. The chance to hit with one against a TAG'd target is exactly the same as an ADA AIV, but requires the TAG in the first place, and can't reach out to the Inner Ring like an ADA AIV. There's not equipment encroachment, and it is arguably a less effective use of an Arrow IV launcher than just bringing ADA AIV in the first place.
The exact text of the book probably needs a minor errata, because I'm sure you won't be the only one to bring up RAW (which isn't a knock on you at all), but the actual issue resolves itself pretty clearly when viewed in the lens of "yeah that seems right".
(If anything, "A POI isn't a unit and therefore Homing Artillery's POI can never be close enough to a fighter to work because the rules for checking distance to a flightpath use the word 'unit' only" fails the 'does it make sense?' criteria pretty hard. Technical writing is hard enough already.)