Exercise Non-Responders?

Are any people real “non-responders” to exercise?  This paper says “no” – those people just need a “higher dose” of exercise in order to see a response. In this case, the exercise was aimed at “cardiorespiratory fitness,“ but presumably the same applies to muscle-building as well. This does not mean, of course, that with a sufficiently high dose everyone will ultimately obtain the same results. Some people are indeed “hard gainers.” But they should be able to achieve some reasonable gains, albeit not at the level of all those “easy gainers.”