We're both using our own discernment here. Steele and De Gea have always been very close and it would come as no surprise if De Gea shared/talked about his frustrations. Steeles comments for me are subtle, maybe even veiled but they allude to what was probably the real reason De Gea wanted out.
LVG was very blunt about his expectations when he joined and some players didn't like it. If you remember a few others left soon after, even fellow countryman Buttner who admitted that LVG didn't rate him. Welbeck was ushered out too without a seconds thought
What I remember is De Gea being excellent from day one and playing the way you would expect a goalkeeper coached by Hoek would. So between that, the links with a return to Spain and his personal life, I would think that Steele is speculating in order to create the idea that everything was fine at United in 2019 more than anything else, which he barely hides in his quotes. I could actually understand your point if De Gea didn't immediately adapt and improve with Hoek but he did.
As for Buttner and Welbeck they have nothing to do with De Gea, I'm not really sure about the point that you are trying to make. No one rated Buttner and righfully so, while Welbeck didn't really deserve a second thought.