I get the impression that the Ukrainians have had a really, really good think and lots of really honest conversations with themselves since about 2014 and worked out what was going to happen next and prepared themselves very well for it.

As a specific example they seem to have regularised and de-radicalised most of the irregulars in Donbass and turned them in to light infantry.

So I would bet that you are entirely correct, that the Ukrainians were expecting to end up in a siege and have prepared themselves for it as best they can.