[I would expect the Ukrainian losses to be higher than that, but as noted, the Ukrainians have an enormous pool of manpower to draw from.]
What the Ukrainians appear to be doing is shoot and scoot, against Russian formations and supply convoys that are strung out across miles and miles of territory. Think steppe raiders in the form of cavalry archers sallying out with their composite bows. Except Ukies are firing one-shot one-kill fire-and-forget man-portable PGM’s. Even if it takes 3 Javelins to completely turn 1 BMP-3 into scrap metal and hamburger scraps, that’s a huge advantage vis-a-vis the bow and arrow warriors of antiquity. Where steppe raiding mounted archers had to venture dangerously close to their adversaries, PGM-armed Ukies can unass the firing point almost before the Russians know they’re there, which has to be hugely demoralizing for the Russians on the receiving end of the munitions.
One of the big advantages of the Mongols and other nomadic raiders was they literally lived off the land. When conditions were good, as during the medieval warm period, their mounts could live off the steppes, and they lived off their mounts, to a large extent. That gave them a huge supply advantage.
The analogy of shoot and scoot is apt. The Russian forces gave up many advantages of armored forces by their inability to leave the roads, and their logistics problems.
It appears they completely misjudged the Ukrainians will to resist. They thought it would be a cakewalk.