As we discussed on IRC, "data race freedom" is equivalent to guaranteeing memory safety, and so, in a kind-of pedantic way, data race freedom is the core guarantee. That said, the original comment would've been better phrased as "one of Rust's core guarantees".

Furthermore, I still think the best phrasing of the rules is for that guarantee: if a rule is removed one can usually construct fairly simple programs that have data races/memory unsafety. Of course, it is definitely true that the arbitrary rules have other benefits, but if there was a simpler scheme that gave the core memory safety without the other things, I think Rust would've adopted it.