Yeah, if you go to the westeros.org wiki their defense of R+L=J https://awoiaf.westeros.org/index.php/Jon_Snow/Theories on this point is that many Targaryans don't have Valyrian traits.  However all those references are from outside of the book series (A World of Ice And Fire, Fire & Blood, The Rogue Prince, etc) which is not very satisfying.  GRRM should have put the evidence into ASOIAF itself.

I've watched a lot of AltShiftX and various other stuff, not sure I've seen Preston Jacobs. I'll go watch a few.