"Just had a partially colored envelope (click on the light switch) so you could see the filament directly. What am I missing?"
No. If you watched that guy's video, as he repeatedly said, the only reason you can see the filament is because of the camera itself. OK, wait, the video was somewhere else... maybe linked to that page, maybe not, but yeah, it's the camera picking up the filament. You could not see the filament.
Satin-Brights were opaque when lit. This picture gives you an idea as to how opaque they were:
The metallicized coating not only gave them that unique, satin look when they were off, it gave the light they cast a very soft, "satiny" glow when they were on.
We had a bunch of them, but not enough to light the whole tree (our trees were always 8 to 10 feet high and quite wide, benefits of living in an old house in a fairly rural area with lots of Christmas tree farms close by).
Friends did have a tree that had all Satin-Brights, only theirs was all gold and blue because the Dad was a retired Navy officer. The effect was absolutely breath taking.