I'm a bit confused; I don't think I have ever seen rebar or even mesh that was not tied together. A simple piece of tie wire holding the pieces in alignment counts ... there's no need for anything fancier.
I have seen isolated bits used when a repair or change was made to an existing pour .... and I've never seen those connected to the existing bar.
I don't think you HAVE to tie the rebar to the GEC, unless that rebar is accessible. For a Ufer, a length of copper wire will suffice - and it need not be in contact with th rebar. The rebar spec only applies, as I understand it, if you wish the rebare alone to be the "Ufer," without any copper wire.
Maybe I'm just confused ...
