I really dislike that argument, to be honest. If we resort to such extreme linguistic positivism that we accept that “words mean whatever each individual wants them to mean”, then language ceases to be a useful means of communication.
This is especially true in technology. Definitions, however arbitrary, are important, because we wouldn’t stand a chance of any sort of precise communication if definitions aren’t fixed. Furthermore, definitions should be useful too – but when they imply effects that can’t (or are at least unreasonably hard to) be proved or disproved, they lose their usefulness; consequently, such a definition is approximately no better than a nonexistent one.
This is why “every possible bug that might ever result from doing X” (the core of what some people might consider “unsafety”) is not useful: it’s so broad, so complex, and so hard to quantify that one doesn’t really gain any information by using it, and this is why I don’t think it’s a good move to allow this overly liberal interpretation. At best, it doesn’t provide any new insights, at worst, it just causes confusion.
My opinion is that if one wants to participate in a discussion about technology (for instance, in this case, to judge the proposition “Rust has memory safety”), they should learn the vocabulary corresponding to the technology at hand, as not doing so only leads to chaos and unnecessary debates. In this sense, yes, dictionaries and textbooks do (or at least should) define at least the part of the language that is concerned with technology.
As a closing thought: it is an unfortunate fact that natural languages are fundamentally unsuited for a 100% correct description of scientific truth… however, I don’t think it means that we shouldn’t at least try our best. Yes, for such a 100% correct and precise description, one would ideally, rigorously and mathematically, formulate Rust’s operational semantics.
That is not a great way to do marketing though, when newcomers just want to quickly know why they should even consider trying Rust. So, as long as we have more to say in detail, and as long as we don’t claim that “Rust prevents absolutely all possible bugs”, I don’t consider the “Rust is a memory safe and thread safe language” tagline the least bit dishonest, misleading, or even just an unfortunate wording. It’s a fine formulation in its own context.