I didn’t look at their implementation, but in general, strings don’t have to be aligned so you can only peek one byte at a time looking for the end, besides not wanting to annoy valgrind and other similar UB detection tools by reading past the end of the string.
Strawman; nul-terminated strings are horribly slow for nearly every application. Hence I assume (especially given that they are using c++) that they are using length-accompanied strings rather than nul-terminated ones.
