I thought in the end the announcers reasoned it was the contact not the slide that resulted in the interference out at first.
If you slide, then there's specific rules about direct line between bases, no leg lift, etc., but if you don't then there's another set of rules which dictate what you can do regardless of whether the fielder tries to make the next play or if it would have mattered.
Not sure I would have realized it as quickly as U2 did in that game, but that's why they're there. If you don't recognize it right away, it's awfully hard to try and sell afterwards.