This is the attitude we should adopt when learning Tai Chi: complete non-attachment to what we think we already know.
Trying to protect children from all discomfort is natural in humans, but it is possible to overdo it. A tough stint makes one strong!
To understand this teaching, it's neccessary to emphazise context. For a beginner, Tai Chi is a complete reeducation of posture and movement.
Tai Chi teachings are not static entities. Didactics employed by a Tai Chi Master will change according to conditions and level of the students.
At the end of a very tiring Tai Chi practice session is when you will face the biggest challenges have the best learning opportunities.