No that is wrong. Currently the x64 port of LuaJIT is limited to about 1GB [1] in the best case and can fail to allocate any Lua objects at all if the lower region of the address space is already completely consumed.
Thanks. A slightly later discussion I found is here[0]. However, this is not a limit imposed by the tag structure used by LuaJIT, but rather the Lua/LuaJIT GC (from a quick reading of the mailing list discussion - I might be wrong here). Apparently, there's a GC in the works that is supposed to resolve this, but it isn't there yet.
[1] http://lua-users.org/lists/lua-l/2010-11/msg00241.html