Without seeing all the video, whilst there is no power to arrest for not wearing a mask, you can be reported for the offence.
If he fails to provide his details then he can’t be reported, so he can be arrested under S24 PACE, so his details can be established and the reported or charged.
Fair enough, although personally I think it's badly thought out legislation. But the copper in the video says,
"You're being detained for not wearing a mask"
Which is clearly not correct.