Can someone with more technical knowledge than me explain why a pilot should be made with unreal engine? Based on experience, I'd guess that the tools Disney has at their disposal are far more powerful. Maybe it was just quicker for a test of concept since Square would be more familiar with that, but I feel like using the tools that they typically use for CGI movies would be far better for an actual series.
Economically, whatever tools Square-Enix can use the best would be the most efficient. Disney proper should be making other content. Overall it just means more content for Disney+ in general. It's kind of like when Nintendo gets Ubisoft to make a Mario + Rabbids game. Even if Nintendo developers could have done a better job, it's still better to have someone else make stuff for their platform, because then Nintendo devs can make other games. In both cases it's just more content.