In 2013, David was officially separated for 2 years, probably more. As far as we know, PM is separated for less than a year now.
But you’re right, it doesn’t really matter. The thing is you can’t compare these two men and their relationship with Gillian. There’s one guy she has a real and complicated history with, with whom something has probably already happened in the past, probably many times, romantically and sexually wise. She knows him by heart, she loves him, she works with him, she’s here when he needs her and vice versa, despite him being married or not, these two have had an on going relationship over the last two decades, either you think it’s been romantic or not. On the other hand you have this man she knows for a few months, she’d briefly worked with ten years ago and probably never heard of him in between. Really, these two don’t stance the comparison.