This is anecdotal, but I currently eat high maintenance calories (more than calculators say I should, but I maintain my weight) and about 30% of the time I do not count calories at all and will consume 2000-3000+ in a day. The day after, I will typically have gained 2-3 lbs. I do not fast or exercise the extra calories off, I just go back to eating high maintenance and usually after a few days the scale is back to normal. This also applied to week-long vacations where I don’t count, if I didn’t go completely crazy the weight comes off within a week of being home and counting again.

Note that it took me years to get to my current routine after recovering from an ED and very slowly increasing the frequency of these “non counting” days to assess how often I could do them without gaining weight.

Of course, I’ve never eating 10k calories in a day, but I believe the people who say the excess weight comes off more or less on its own over time once they return to their normal routine. It’s easy to say it’s impossible, but metabolisms and nutrient absorption vary a lot by person. Data about TDEE, CICO etc is based on averages, not individuals, and there will always be outliers.