It's true because we are most often sitting in easy pose and not jumping into chaturanga or kicking into handstand.
It's not true because we have many extremely pressurized exercises -- holding our arms up for 11 minutes or more, for example.
These exercises are part of the reason Kundalini Yoga works for me. Granted, I'm not excited when I'm the student and one of these comes up. I go through a whole mental process in those 11 minutes. It can start with discouragement (sometimes internal cursing). Then I shift into a silent pep talk - "I know can do it. I've done it before. I'll be so happy I kept up. Be honest. Not lazy." That mental state morphs into a determined commitment, and every once in awhile, commitment transforms into a euphoric feeling that I could stay in it forever.
It's become clear to me that I need to go through that process, so that I can experience my strength and remember that I am strong -- that I can conquer my mind. Some of us need the intensity of this practice in order to be fully present for a spiritual experience.
Yogi Bhajan's words tell us the greater purpose of these powerful exercises:
"I know that without pressure, carbon can never become a diamond. If you think you cannot stand the pressure, then you do not know ... you are a human being, because the human body... (has) been made to withstand extreme pressure to crystallize the consciousness."
To crystallize the consciousness. To reach our highest. To experience that part of us that is beyond the thinking mind.