Surveying people is pretty important in a study like this, and valid science.

Regardless, if you read the paper, you'd learn that they did do objective measurements, namely etCO2 levels and bio-markers from saliva.

Of note: "when etCO2 fell below approx. 20 mmHg, it was virtually guaranteed to trigger at least some (and often a strong) departure from ordinary waking consciousness. This effect is particularly intriguing because in non-breathwork-related circumstances, an etCO2 of 20 mmHg or less would be considered a sign of severe physiological malfunctions, e.g. of the heart or lungs"

But anyone who tried breathwork, especially in group settings with accompanying music, knows such practices can lead to intense trance-like states.