I wouldn't say the reason so many claim him is because they have learned through his video series, but that they learn through his video series so that they can claim that.
But actually, many of those I've seen who claim to have studied with him or to be his disciple don't actually do the forms like him at all. Whether like the video versions or not. Some just claim him for the fame of his name, even if they have legitimately learned elsewhere.
But there of course are those who teach the material straight from his videos and claim lineage with him. Which is hilarious because a lot of the forms are purposely screwed with on the video series. They either leave out important postures, sections, change directions, or do something completely different.
I've seen a couple people claiming lineage under Shi Deyang or Shi Suxi and they do the forms with the missing move right in the same place as the video, which changes the direction of the entire form, or they are missing the precise section left out of the videos.
They then have to make up their own applications, and I hope they aren't using the ridiculous applications Shi Deyang did for his 'Actual Combat' video series, lol.
I don't know. What can I say?