I don't know of any code requirement for plumbing and electrical to be 12" apart. Recessed fixtures require more room than junction boxes, so perhaps the plumber was trying to accomodate the electrician.

" the plumbers said they had to make sure they were at least 12" from the electrical... "

Unless it is a local requirement it is not in any of the model codes.

As long as the wring does not touch hot water lines there is no issue (and fished wires often touch anyway, it is not a code requirement.)

90C nonconductors vs. 120 F water is not a problem.
Steam lines (100C/212F) or hot hydronic lines (180F) maybe.

PEX may not be run within one foot of a lighting fixture...that is the only code requirement that I am aware of.

Thanks Lazypup! It must have been that they were lighting fixtures.

So, it sounds like I can run my wire in the wall to light switches and outlets without issue. (Plumbers will be here again tomorrow, and I can verify it isn't a local issue, but at least my brain can finish planning the placement of all my boxes without having to figure out how to stay away from the pex).