I noticed it myself with cycling. Took 8 years off the bike, when I started up again I was nearly back to my old FTP in about 2 months despite starting from basically zero. Muscle memory is real, where I am now as a returning cyclist would take a pure beginner cyclist at least 4+ months to get to, fitness wise.
That said, you do have to work somewhat hard to maintain. With cycling, just 2 weeks off the bike is enough to see a VO2 max drop of anywhere from 4 to 7%. After just 4 weeks, your glycogen storage capacity decreases and you start rapidly losing fitness. After 2 months, you are basically now out of shape.
Detraining happens faster than most people think. And therein lies the danger with over reliance on LLMs for your cognitive skills. Detraining there happens just as fast, skills atrophy in a matter of weeks, not months or years.