That is a pretty bad video, with the constant moving and shifting and zero footage taken from where we ask for it, the rear oblique. I'm sure you've read the sticky in all it's glorious detail, and could get an A on it in a book report.
Lumbar flexion is not caused by a recent DL workout. It's cause by your inability to control your lumbar spine. Have you watched the video here on low back position control and read the relevant section in the book? Can you independently control your lumbar spine without moving your chest or upper back *at all*?? Is your low back flat when you deadlift.
It also looks like you need to lean over a bit more. Then, make sure you KEEP your chest up as you drive your hips, without lifting your chest and making your back angle more vertical. Just keep it as one rigid unit that rises exactly in time with your hip drive.