The assumption is that he didn't know Hardy beforehand. Maybe he did, maybe he didn't. By the way, there is no mention in the article that Hardy was the heir apparent to Pop. Could be that Ham got that one on his own.
Edit: This article. Right in the headline, don't even have to click on the story when it does come up in Google:
https://celticswire.usatoday.com/2021/10/09/nba-boston-celtics-hardy-to-spurs/