Starting Solids?

At our 4 month check up the pediatrician gave us the green light to start solids. But our little guy cannot sit up at all by himself. Like yes he sits on our laps well, but he’s nowhere close to sitting up unassisted. And when I said that to the pediatrician he said it’s okay one of you can hold him and the other can feed and that this will help with his caloric intake (he is 73 percentile weight and 83 height and on his curve) and help him get off formula sooner.

What’s the rush?? I’d understand if he was having trouble gaining but it feels unsafe and rushed. Everywhere I’ve read says to wait until they can sit mostly unassisted. What’s everyone else’s thoughts on it?