It is very possible that my interpretation of Hoare's talk is flawed. It was a very unsatisfying talk, and I got the impression that he wasn't really expecting to give it and so wasn't really prepared. He said something about expecting to be part of a panel of people all sharing their "greatest mistakes" and went to some effort to get people in the audience to contribute - to argue for or against - to make it a more substantial discussion. So it would almost certainly be unfair to assess Tony Hoare's really thoughts based on that talk.
For the moment however that is all we have, and to my mind it came across a lot like the proverbial dot-com-boom business plan which you might remember as:
1 - give stuff away for free
2 - ....
3 - profit.
In this case it is
1 - Invent the NULL pointer
2 - ...
3 - Cause a billion dollars of damage.
In both cases there is no clear link from the first step and the third step.
I can happily agree that Tony Hoare invented NULL, and I can happy agree that programmer errors that manifested as dereferencing a NULL pointer have had a large cost, but I cannot see how the one leads to the other.
It would seem equivalent to blame Marie Currie for the current serious radiation problems in Japan. She coined the word "radioactivity" and radioactivity is causing serious problems and endangering lives. But it is hardly her fault.