This is BS. It takes a couple seconds to check a blindspot. Same with most other things you look at, navigation, radio, whatever. The rest of traffic doesn’t wait around while you are focused on that one thing.
I think his point was that the time to check the lane over is before you initiate a lane change, not after. About the only reason to initiate a lane change without pre-checking where you're going is if there's an emergency (like the incident we're discussing) and then it's questionable whether it's better to do the lane change anyway...