The first quote about slow eccentrics is not done with heavy weights. We are talking about 6-10 sec eccentrics here (even up to 30 if you use the darden method). Hence you don’t create as much muscle damage and focus more on the muscle fatigue/ growth factor pathways. These are easier to recover from. However, training muscles eccentrically will still put a lot of stress on the muscle fibers because less motor units are recruited to resist the weight when the muscle is being stretched.
I don’t know where the second quote came from as I don’t have time to search through the article. In what context is it presented? If you de-emphasize the eccentric, the focus lies more on the isometric or concentric. For muscle hypertrophy these contractions are mostly used with medium weights (relative to the strength of each individual) to stimulate the growth factor/ muscle fatigue pathways as well as they will cause a lot of metabolic stress, which is also a precursor to muscle growth next to mechanical stress.
I believe what CT meant whas that true hardgainers need a different hypertrophy pathway than muscle damage (mostly stimulated by movements where the target muscles are being stretched under heavy load or explosive movements done with heavy weights) to cause their muscles to grow because muscle damage takes the longest to recover from. Offcourse there are a lot of other factors involved as well but when it comes to training this can make a world of difference.