Since the assembler sources of strlen for AVX2 in GLIBC are virtually obfuscated by the macros, here's the actual assembler: (comments are mine)
Dump of assembler code for function __strlen_avx2: (obtained with GDB disassable)
0x00007ffff7bf27e0 <+0>: endbr64
0x00007ffff7bf27e4 <+4>: mov %edi,%eax
0x00007ffff7bf27e6 <+6>: mov %rdi,%rdx
0x00007ffff7bf27e9 <+9>: vpxor %xmm0,%xmm0,%xmm0