Well, giving a base to a mercenary organization that is so far off the kilter that they revolted against their own country just to do another 180 turn is peak non-credibility.
I think luka thinks that it will make him safer from the polish invasion he is expecting and/or him being thrown out of the window by his own people. I keep thinking that if Prigozhin gets annoyed he might be the one throwing luka out of the window.