Absolutely not true. There seems to be quite a few urban legends surrounding CA smog legality.
I would suggest researching the CARB website for Executive Orders (EO's) that pertain to state approved aftermarket parts. There are thousands of them, including for replacement of catalytic converters & related piping.
If you can't find what you're looking for, contact CARB directly via e-mail & ask them for a specific make, model, year & modification. They are usually pretty good about returning such inquiries.
I know from experience that 5.0 Mustangs, F150 trucks, Crown Victorias & other similar Ford vehicles that originally came with 4 converters can have them replaced by 2 converters via an X or H-pipe per two separate CARB EO's.
I had to jump through this hoop when I registered my '89 Saleen with a
BBK H-pipe.