What is yoga?

YOGA:

literally “yoking”;

To perceive the constant union, unity, or copulation with the divine self in every breath, in every moment, in every activity, and in every achievement.
Yoga is the way one can perceive divinity manifested in the whole universe as well as in the entire body.              

Paramahamsa Hariharananda
in, ‘The Bhagavad Gita, In the Light of Kriya Yoga’ - Book Three

Surprising about this?

It depends on how familiar you are with yoga.
If you are familiar with the original yoga tradition, then the description of yoga as being "to perceive the constant union, unity, or copulation with the divine self in every breath, in every moment, in every activity, and in every achievement" won't surprise you at all.
If on the other hand you've only come across yoga in gyms and fitness clubs and acrobatic demostrations then the definition above might surprise you.

Peter Littlejohn Cook

Mestres de Kriya Yoga