Eh, a string 'is' its semantic definition and its in-memory representation and I'm sure other things in other contexts. Multiple notions of the same thing coexist outside of computers too. I might be a bunch of cells and physiological systems to someone in biology or medicine, a legal person to the state, something else to a philosopher.
Something like a language specification would have good reason to stick to talking about the semantics, but that isn't a reason to discard any source that uses 'is' to talk about a different aspect. For example, Go's authors have written explanatory posts both about the semantics of strings and their in-memory representations, and it would be weird to say one of those posts is Just Wrong if it says 'A string is [whichever view of strings is most relevant locally]'. Sometimes another way of looking at the system is clearly better for the immediate purpose (writing about memory-use optimizations you would need to know what's in memory) and sometimes discussions need to jump around and look at the different levels at different points (like here, where the post both fills folks in on interactions between various APIs and performance, which is a(n important!) aspect of the implementation).
Something like a language specification would have good reason to stick to talking about the semantics, but that isn't a reason to discard any source that uses 'is' to talk about a different aspect. For example, Go's authors have written explanatory posts both about the semantics of strings and their in-memory representations, and it would be weird to say one of those posts is Just Wrong if it says 'A string is [whichever view of strings is most relevant locally]'. Sometimes another way of looking at the system is clearly better for the immediate purpose (writing about memory-use optimizations you would need to know what's in memory) and sometimes discussions need to jump around and look at the different levels at different points (like here, where the post both fills folks in on interactions between various APIs and performance, which is a(n important!) aspect of the implementation).