The TL 15 equipment - battledress, fusion guns - is really expensive, so I'd expect the number of troops equipped to that standard to be limited.
IRL, the ability of modern armies to fight a major protracted war using only state-of-the-art equipment is limited.
We see in the Ukraine war both sides relying heavily on TL 6 gear, and even TL 5 gear. Both sides have state-of-the-art systems, some of which are too expensive to field in numbers (for example, modern MBTs), and others which are becoming the mainstay of the conflict (drones) The Russians used a ship Kommuna commissioned in 1915 to recover items from the sunken Moskova. Maxim guns and Mosin Nagant rifles and howitzers designed in the 30s have seen combat, and tanks, APCs and IFVs from the 50s and 60s are common. The Ukrainians had lend-leased Thompson submachineguns in stockpile, unfortunately captured. 1950s anti-tank grenades - almost suicidal to use as designed - have been repurposed as drone bombs.
IRL, we are in a situation where the current tech is so expensive that only a limited number of people can have the best equipment. In the Iraq invitation, Donald Rumsfeld famously said "you go to war with the army you have" in response to criticism that some service members had Vietnam era gear. And this was not even an all-out war.
In a FFW situation, the Zhodani need to transport their soldiers to the battle and that transport capacity is the bottleneck. I expect they'll be equipping them to a high standard and those they can't afford to equip to TL 14 can guard the rear.
I don't find it puzzling at all that not all Imperial units are equipped to TL 15. There aren't that many high tech worlds in the Marches, or at all. I would expect, however, that there will be TL 15 stockpiles at bases, and probably also TL 14 gear, and that they will have back-compatibility built into their gear and doctrine, in order to be able to use lower tech gear as well, for example in ammo and spare parts. Certain critical types of assets might be kept at TL 15 as long as possible - as enablers to make the lower tech equipped units be more effective. Lower tech gear would be used where it doesn't matter so much, i.e. when the high tech doesn't give much advantage. High tech units' capabilities might be leveraged to make the lower tech units more effective, and this would no doubt be practiced and written into doctrine.
So I would expect that units wouldn't just have a single TL level and no gear of higher tech than that. Really important gear will probably always start out at TL 15, even if the rest of the unit doesn't have that tL, though maybe it will be replaced by lower tech gear later in the war.