It is a very very raw estimate without much usability. VO₂max at biking cannot be truly estimated without the power meter, because there are far too many variables playing a role. At running, the pace, HR, HRV, and the respiratory rate, and the terrain profile are sufficient for a relatively accurate estimate, but a cycling the true performance is much more influenced by external factors - road surface, type of bike, size and type of tires, their pressure, gear usage, your sitting position, wind, brakes, clothes, etc., etc. - hence making the simple estimate without the power meter is rather useless. If you look at your pace and HR, you get about the same level of information.
