Nope, not true. I looked through a lot of the old forums for discussion on when the UCI is generated, it is very common for UCI to be assigned at some point post ITA. here is an example:Hello everyone! Please help me asap. I have created my profile in june 2017 with 387 CRS which had an UCI. Again I recreated on September 2017 with 438 score. But this time, it had no UCI. Is it true that ITA will not be sent for the profiles with no UCI?
https://www.canadavisa.com/canada-immigration-discussion-board/threads/unique-client-identifier-not-showing-on-my-application.517566/#post-6358002