Possibilities include 1) he was unaware of it, and 2) that he didn’t consider a Russian lawyer whose clients included a state-owned business to be a Russian government official. Or 3) that he lied about it. But the first 2 are in play as well.
Assuming that they’re lying about it, it’s not hard to understand either. They are being hounded about every contact with any sort of Russian person, and think they’re being persecuted unfairly. Denying things that are likely to come out anyway is not the smartest approach, but it’s one that comes naturally to many people.