That's not true. I've had mandarins for years. A home aquarium will never have enough pods to fully sustain a mandarin.
I feed them nutrimar ova (shrimp eggs), then move them onto mysis. Then they'll pretty much eat any frozen stuff that fits in their mouth. I had one that used to eat flake food too.
Main thing is initially giving them a lot of attention with spot feeding small food and moving them onto larger, more meaty foods over time. Then I just feed them once a day with the rest of the fish, no spot feeding. They're only tough because people don't know how to care for them or they're not willing to do what's required.
I have a target mandarin living in a 10g. Got her as a minnow. She's about 8 years old now. I have to shoo her away from my corals when I feed them because she is a pig. She eats everything.
My 2¢