Anyone can play a video without even watching it.

And anyone can watch a video without learning anything. Even if you sit there and and look at them watching it.

So maybe give them an in class quiz on what was in the video?

This is a strange one. As Jeff said, students could get this for free by putting the video in a background tab and muting it.

Maybe consider changing how you assign practice in KA. I've had most success assigning minutes rather than points. Students need to do x minutes per week outside of class time. I emphasise that what I'm really after is skills improvement rather than correct answers in KA. I don't really care that they got correct answers in KA, I care that they can cope with the question I give them to check their skill level. By doing it this way, they are rewarded for learning rather than the activity so there is less motivation to "cheat" because the extra points isn't something I value anyway.

Of course that doesn't always work as some students still care a lot about the points no matter what I tell them. I had one who left a video on repeat overnight just for the points even though we discussed it and she knew I didn't care about the points - she just liked to see the number go up.