What he actually said is factually correct. You even agreed with him, probably didn't realize it, in your last paragraph.
He will be fine, but for this year, he is not physical enough to play the 4 in that conference.
And as far as driving by people, etc. It will be much harder for him to do when he is pushed out 5-10 feet further from the basket because he is not yet strong enough to establish position.
This is the correct take. However, the ISU poster he was responding to was making it out as if White will NEVER be a legitimate Big Ten 4. He's thin for now, but he'll fill out as his career progresses. But yes, he's going to be up and down this year as he adjusts and gets out-muscled at times.