As other have said, this is no longer considered to be "true".
Yes, they make it harder to get memory corruption. But that's a very narrow definition of "safe".
If you tell the computer to copy a string, or append to a string, it's wrong, and not safe, to only do a partial copy or concatenation.
It's a huge footgun. Smaller than strcpy(), sure, but "safe" it is not.
I don't really have a better answer for C.
The better answer would be to add data types like SDS[0] to the standard library, and use them as ADTs (Abstract Data Types) [1].
Unfortunely WG14 has proven in 30 years of existence, that it isn't something that they care to fix, and while 3rd party solutions exist, without vocabulary types on the stardard library adoption will never take off.