You're right, the qualification appears to be unnecessary there.
Don't try to learn C++ from standard library code, it has to be written in a very peculiar style for various reasons that probably don't apply to you, including backwards compatibility, portability, alternative configurations (e.g. disabling all namespaces for historical compilers). It's possible that in some configurations _STD
expands to something different, or it's possible that it's just redundant (but harmless.)
what would be the name lookup rules for names in a template functions with arguments ? I think that a qualified-id would look directly for names already declared without ADL but an unqualified name would do an ADL, am I right ?
Yes, ADL only applies to unqualified names.