Robertson is a false teacher but he's right about Yoga being dangerous. Not because of "the mantra" but because the entire practice is occult.

Yoga is closely tied to the occult doctrine of chakras, multiple energy points connected to consciousness that exist in different areas of the body. For example Kundalini Yoga doctrine teaches that through Yoga, a coil of energy located at the base of the spine (which is sometimes referred to as a the Kundalini Serpent by Luciferians) can be released, traveling through the chakras and eventually releasing deep self-awareness. This of course is occult. Trying to awaken a "serpent" inside of you so that it can travel through your body and give you deep awareness may not be what the average Westerner thinks of when they attend their Yoga class while sipping on Starbucks, but that doesn't mean it's not real.

Even the wikipedia page for Yoga backs this up: "In the 1980s, yoga became popular as a system of physical exercise across the Western world. Yoga in Indian traditions, however, is more than physical exercise, it has a meditative and spiritual core. One of the six major orthodox schools of Hinduism is also called Yoga, which has its own epistemology and metaphysics, and is closely related to Hindu Samkhya philosophy." Here's an enormous statue of the Hindu god demon Shiva practicing Yoga in the Padmasana posture. I wonder how many people have held the same pose and tried to clear their minds, thinking it was just harmless exercise.

Some may believe they are able to separate the spiritual side of yoga from the physical side, but I am of the opinion that it is not to be touched. All Hindu gods are demons, chakras are occult, and yoga is a carefully crafted form of ancient Hindu worship.

Don't take my word for it, wikipedia again seems to agree with me.