"Proto-Germanic had no ā"
Proto-Germanic ā is a very late development, and occurs only in a handful of cases. The change of post-PIE ā to ō is well established.
But if that's an accepted development, why would an apparently respectable linguist (from a quick google of the author of the source provided) still make that claim? As a matter of fact, the source states that this is "generally believed" to be a "migratory term", which would thus have been able to be borrowed from an Indo-Iranian language (unlike what the IP's edit suggested, Bandle does not claim it is a direct borrowing from "Old Iranian", by the way, just that it is of Indo-Iranian origin). I have no idea what Bandle bases his conclusion on, but given his aforementioned apparent reliability as a linguist and his reference to the existence of a scholarly dispute on the matter (he explicitly mentions *mēkijaz as well, saying that that reconstruction has "long been disputed"), perhaps it at least merits a mention on the entry.
I don't see why it would merit a mention if it violates a well-established sound law of Proto-Germanic. It would be similar to ignoring Grimm's law. If we are going to include various proposals in the entry, we should at least filter out the ones that are implausible from the outset. If you really want to discuss this more, I suggest WT:ES as I don't really have more to add.
Ok so I checked the source more attentively and I think the anon who edited the entry may have misunderstood the author's position, with me following their error: Bandle claims that the prehistory (not the reconstruction) of *mēkijaz is disputed, and that the "Ancient Nordic" (Proto-Norse, I suppose) form would be *mākijaz -- which is also more in line with sound changes as I understand them.
The claim about an Indo-Iranian origin is for the PGmc. *mēkijaz, then: apparently it would be be a migratory term ultimately from Old Iranian maδyaka or some closely related form, meaning "something attached to the middle/belt" which would be etymologically related to *médʰyos. Would you agree with the addition of that theory to the entry?
Either way, this just goes to show that I shouldn't try to etymologize too much on three hours of sleep, sorry about the misunderstanding!
It still seems a bit far-fetched. Why was it not borrowed as *madjakaz instead? Why was the word transformed in such a major way? Also, *hanapiz suggests that Iranian borrowings occurred before Grimm's law, so you'd expect *madjahaz or *madjagaz, or perhaps even *matjahaz or *matjagaz. None of those resemble the actual word.