Their issue isn't against "praying for what you want" so much as the angle it is coming from. In this context, they describe a practice that is not praying to God, but rather praying on behalf of what one wants. On a deeper level, I can pray to God for what I want, rather than "the universe", but it is not what I think of as "Christian".

God knows what will allow me to fulfill my most important purposes, and that is what He will help me do if I ask and work for it. If I pray for a job and He delivers, it isn't because I wanted it. It's because I made a covenant to care for my wife and children, and to fulfill His command to multiply and replenish the Earth.

"Manifesting" for what I want to the universe is asking the wrong entity for the wrong thing. Seeking what God wants and trying g to align myself to it is Godly. Seeking what I want, without that alignment to God, is giving in to the carnal mind.