@FP why would there be no elements of some serious formations there? What would have supposedly happened to them? It's not like we saw them active elsewhere around that time.
How silly is it so assume 3 battalions of Rosgvardia was holding a 100 to 150KM long front line?
That's what the info we got said. Vague though it was. That some 1500 Russian forces were overwhelmed by 7k, 8k or so Ukrainian forces. Maybe there were a few tanks, but I don't think we're even talking about a full battalion here.
If there were more Russian forces there and they had armour support, and presumably helicopter gunship support and artillery and so on - then they would have managed to resist the advance, at least for enough time for Russian to send reinforcements there. That's self-evident. And certainly this Ukrainian tactic of using HMMWVs, pickup trucks, and small-unit advances to penetrate through wouldn't have worked.
The other thing is, and this is confirmed by various sources - is that the Russian defense in the Kharkov region was based on a system of checkpoints along various roads.
Not on defensive lines of deployed formations, along with entrenchments, dug-ins, and so on, as one might expect on a front-line. This again indicates that the Russians had only light units there such as the Rosgvardia, who deployed only as they were trained; not some armour-tank, motor-rifles and so on.
As to why this would be - well it could be as it was supposed, that Russian forces were in the process of withdrawing from there. I'm not sure it makes sense either, the article after all goes into the importance of holding the region and Izyum for the process of encircling the Ukrainian forces in the Donbass.
But as outside observers we can only judge based on the facts we see plain as day. All conjecture and speculation on the objectives of the Russian military at that stage, or the apparent importance of this or that area to them - is just that.