And he would really know that if he had ordered their murder years ago in the tower.
Except the princes vanished more than two years before Henry even came back to England, while he was still in exile in France.
Richard suffered substantial bad PR over the princes' disappearance and lost significant support over the rumours that he had killed them. If they were actually alive and well during the two years between their disappearance and the Battle of Bosworth, then why wouldn't Richard just bring them out and prove they were still alive? Would you really let people openly accuse you of child murder - placing both your life and position in jeopardy - if you had the children you were being accused of murdering safe and well in your custody the entire time? And if they didn't die until after August 85, where they heck were they during those 'missing' two years?
It's just not a theory any historians take seriously, afaik. Henry Tudor was brutal and ruthless after he became king but not only are there no facts to support this theory, the facts actively work against it.
So not clear why it is being trumpeted as something new. ie that Richard was not the evil character that the Tudors wanted everyone to accept as having murdered his nephews (and charicatured by Shakespeare to ingratiate himself with Elizabeth I)
Shakespeare's play was unquestionably Tudor propaganda, but Ricadians were utterly convinced that Richard didn't have anything wrong with his spine and that all that was anti-Richard propaganda and lies too, and then the discovery of the skeleton proved that Shakespeare was right and it wasn't something he'd made up. At the very least, Richard had the Queen's brother killed and stole his lands, drove the Queen into religious sanctuary because he gave her reason to fear for her life and the lives of her children, and kidnapped his nephews and had them declared illegitimate so he could steal his nephew's throne. He wasn't exactly a great guy and he was clearly willing to do bad things in order to become King. That doesn't prove he murdered them, but he's a much more likely candidate that a very religious woman widely described as kind and gentle, or a bloke in exile in another country for two years.