New Rheem heat pump not keeping up

I had a new unit installed in August by a small private company (owner operator).

Now that it’s winter, temps have dropped significantly and it’s not keeping up. It’s 68 on the house and I have it set to 71. Unit is a rheem RH2TZ3617STANNJ

He’s offering to come and “upgrade” the heater pack, saying it’s just the cold weather and heat pumps don’t work that well in such cold temps (it’s been low teens outside). I have a suspicion that they actually never installed a heater pack in my unit and tried to pull a fast one on me. He says that the heater pack is weak.

I’m pretty handy and know a little about all the trades. I watched the install and was home. But i did not witness him put in a heater pack. I opened the panel on the air handler and i feel like it should have gone above the blower in that blank spot. Thoughts ?