What precisely is that definition of memory safety? I'm not sure I follow.

Probably the shortest statement of Filip's definition is at https://fil-c.org/invisicaps . There's a few other statements around HN and twitter, like this https://news.ycombinator.com/item?id=43195623 , and the recent GISMO talk

But you can see in a number of his public comments statements that Rust is not memory safe by his definition because it has unsafe as an escape hatch. https://x.com/filpizlo/status/2079244258062766177 and https://news.ycombinator.com/item?id=49053608 as some examples.