It is the context here that makes the use of stock footage problematic.
Their argument appears to be - Diana was hounded by the paparazzi and media after she entered the Royal Family and this ultimately led to her death. Meghan has been hounded by the paparazzi and media after she entered the Royal Family and they are worried the same thing will happen to her.
They use images of Diana being hounded by the paparazzi
They then juxtapose these with modern images of the paparazzi - grouped taking photos and chasing a target. The natural assumption, given the narrative is that the target was Meghan. This has been demonstrated to be false.
An equivalent here would be, for example, a documentary about Tiananmen Square. With an argument that current circumstances in China are potentially leading to another Tiananmen Square. Then using real images of Tiananmen Square and then juxtaposing those images with modern day images of protests and police force from another country.
It is disingenuous and dangerous. It is seeking to pull the wool over peoples eyes to fit their agenda.
If their argument is valid, use valid and truthful imagery.