Traditionally, I'd say alternator
Back in the old days, when your headlights would pulse like that, it usually was an alternator problem.
Does the flickering change with engine RPM?
How much longer does the CPO warranty last?
Maybe they are hoping it will fail out of warranty.
I presume they tested the alternator and the battery, right?
Ask to see the TSB on this. If the lights are flickering without the heated seats on, something is up.
I'd suspect the alternator.
I'm not too surprised the warranty monkeys can't figure it out, though. Excuse me, I meant to say "highly skilled and trained dealer service technicians"