Are you 100% certain it was on tape? I have read about Diana making the claim that the royals were "reptiles," but I don't recall ever actually hearing her say that.

Diana had a friend who was into the spiritual side of things, I believe it was. This woman reported that Diana confided certain things to her because this woman was more open to ideas due to her interests. It was this woman who made the claim that Diana had said the royals were reptiles.

Here's the thing, even if Diana had said this, she could have meant it figuratively and not literally. It would be like saying, "Oh, that person is a monster!" or "She's such a witch!" None of this is meant to be taken literally, it's just a description of their behavior.

Ah! I found it:

"While researching this book I was introduced to Christine Fitzgerald, a brilliant and gifted healer, who was a close friend and confidant of Diana for nine years. Because of Christine’s understanding of the esoteric, Diana was able to talk to her about matters she would not dare to share with anyone else for fear of being dubbed crazy. It is clear that Diana knew about the true nature of the royal family’s genetic history and the reptilian control. Her nicknames for the Windsors were “the lizards” and “the reptiles” and she used to say in all seriousness: “They’re not human”. There is a very good reason for Diana using this description of the Windsors."

https://prepareforchange.net/2015/07/27/what-princess-diana-knew-the-official-resistance/

So, it would appear that this is second- or third-hand knowledge, in this particular instance, and Diana could very well have meant it figuratively.