If you occupy territory, you cannot normally forcibly eject people from the territory other than in very particular conditions – that’s a Geneva Convention violation – but letting them evacuate at their option is fine.

Given that there’s quite possibly going to be fighting going on through the area, I’d imagine that a number of people might want to do so.

EDIT: Actually, an occupier of territory is normally expected to provide option to evacuate, absent certain conditions, so Ukraine making the offer is in line with the Geneva Conventions:

https://www.ohchr.org/en/instruments-mechanisms/instruments/geneva-convention-relative-protection-civilian-persons-time-war

Geneva Convention relative to the Protection of Civilian Persons in Time of War

Article 35

All protected persons who may desire to leave the territory at the outset of, or during a conflict, shall be entitled to do so, unless their departure is contrary to the national interests of the State. The applications of such persons to leave shall be decided in accordance with regularly established procedures and the decision shall be taken as rapidly as possible. Those persons permitted to leave may provide themselves with the necessary funds for their journey and take with them a reasonable amount of their effects and articles of personal use.

If any such person is refused permission to leave the territory, he shall be entitled to have such refusal reconsidered as soon as possible by an appropriate court or administrative board designated by the Detaining Power for that purpose.

Upon request, representatives of the Protecting Power shall, unless reasons of security prevent it, or the persons concerned object, be furnished with the reasons for refusal of any request for permission to leave the territory and be given, as expeditiously as possible, the names of all persons who have been denied permission to leave.

The part restricting forcible deportations – not what is being done here – and inserting your population into the occupied territory (which, I might add, Russia has pretty vigorously violated in occupied Ukrainian territory):

Article 49

Individual or mass forcible transfers, as well as deportations of protected persons from occupied territory to the territory of the Occupying Power or to that of any other country, occupied or not, are prohibited, regardless of their motive.

Nevertheless, the Occupying Power may undertake total or partial evacuation of a given area if the security of the population or imperative military reasons so demand. Such evacuations may not involve the displacement of protected persons outside the bounds of the occupied territory except when for material reasons it is impossible to avoid such displacement. Persons thus evacuated shall be transferred back to their s as soon as hostilities in the area in question have ceased.

The Occupying Power undertaking such transfers or evacuations shall ensure, to the greatest practicable extent, that proper accommodation is provided to receive the protected persons, that the removals are effected in satisfactory conditions of hygiene, health, safety and nutrition, and that members of the same family are not separated.

The Protecting Power shall be informed of any transfers and evacuations as soon as they have taken place.

The Occupying Power shall not detain protected persons in an area particularly exposed to the dangers of war unless the security of the population or imperative military reasons so demand.

The Occupying Power shall not deport or transfer parts of its own civilian population into the territory it occupies.