this is only true if you passed your natural genetic limit.
This is why i think its stupid to wait till you have been lifting for 10+ years when your basically at your natural genetic peak to cycle.
Bulking as a natural is not stupid, but after 3 years your gains are basically like 1-5 pounds of muscle a year. so yes, try to aim for 1 lb a month after you have been lifting for 3 years... or else it will be dumb to bulk for more than a lb a month... you will just gain fat
or better yet, cycle after you have been lifting for 2-3 years and you will be ok and most likely keep all your gains. YOU ONLY LOSE ALL YOUR GAINS IF YOU PASSED YOUR NATURAL LIMIT. thats why theres nothing wrong with cycling after lifting for only 2-3 years because you are not at your natural limit and will keep all your gains except water weight.
if you lose your gains and your not at your natural genetic limit, its because you dont know how to train or diet properly which most people dont. so yes you will lose your gains.
I know many people, myself included who continued to make after our first cycle. because we have not been lifting for long and were not at our natural limit after our first cycle so we wont lose our gains... but if you have been lifting for 10+ years and decide to cycle and get passed your genetic potential, than yes you will slowly lose all of your gains... if you have no anabolics inside of you.