There are factory variants, the WoB ones. They may not have been seen until attacks on the Protectorate proper began. As for field mods, I suspect that the statement refers to in field modifications rather than factory produced packages.
Not only not seen, but also not put into production until 3072. Even 3058U's writeup talking about "ugly surprises" in the form of upgraded Zhukovs are premature.
As for why the Word would build 'stock' Zhukovs in the '60s? (since it was mentioned earlier) Easy, I mentioned it in the article. Export. The Word was trying to hire merc units everywhere they could, and luring in units with new equipment was a common practice- several Project Phoenix designs are the result of this practice. A platoon of Zhukovs might not be flashy, but they're solid armor units, even in the early Jihad- so as a 'bonus' to a merc unit, they'd be a pretty good way to sweeten the pot. They also are ideal for the Protectorate Militia- cheap, homemade, tough as nails, and effective still even despite their low tech level. And on the defensive (as the Militia was intended to always be), the tanks wouldn't be likely to proceed far from supply lines-which means that pitiful ammo load is less of a problem than it would be otherwise.
The WoB was known for giving merc units hightech units in order to buy loyalty, not just units. Either as a bribe or as an attempt to force them into a company store. Considering the 3060 Battlefield, I'm not sure how much of a bonus 3030 era unit would've been seen as. Sure, its nice to have 4 new tanks, but its going to suck when your opponent out-ranges you, using a common upgrade that's been around for a while.
The WoB was also willing to spend on the Protectorate Militia. Each regiment got a battalion of Demons, I don't think a non-insane upgrade of the Zhukov was really going to break the bank. As for tech issues, the Protectorate Militia seemed pretty hightech in other regards.
It just doesn't make much sense to me. Maybe if it had been the only design, sure, you could make some sort of exception and say it got overlooked. But we're talking about the Zhukov, the Schrek, the Behemoth, as well as the Rhino, and if you want to push out to Epsilon Eridani, the Brutus as well. You can also add in the Puma if you want, which is also produced on Terra, but remains at an Introductory tech-level until 3069 (The Puma also sort of kills the idea of the Protectorate Militia not being able to handle advanced Tech, since we're talking targeting computers, ERPPCS, advanced electronics, B-Pods, Heavy PPCs, MMLs, etc).