Windows XP x64 Edition was released for AMD64 around 2003-2004 before Intel’s x64 chips were out, the only CPUs were Athlon64.
In a way, yes. It used the kernel from Server 2003.[1] Server/client kernels were unified for Vista SP1 and Server 2008, and not sooner just because the 2008 release lagged behind Vista's debut.
Prior to Vista, this kernel difference meant drivers for Windows server and client often had to be different builds. An anecdote that I recall was that although xp64 and 2003x64 could share drivers, it was typical to see drivers "not provided" for xp64, so the people who wanted that kernel just ran Windows Server as a desktop OS instead.
1: https://en.wikipedia.org/wiki/Windows_XP_Professional_x64_Ed...