Earlier quoted context omitted.
Keto relies on being in ketogenesis, which is affected by intake. You can certainly have a "cheat" meal of higher-than-normal carb intake, but you might get knocked out of keto for a few days. If you cheat every week, you could be screwing yourself out of progress.
For keto in particular I'll agree out of ignorance. I was speaking broadly of "dieting makes social occasions hard" which is a common excuse to avoid certain diets. However, even with slower progress, I contend that slowed progress is still an improvement over no progress.
Any progress at all is better than no progress. If being able to deal with social interactions by utilizing a cheat day helps stick to the diet -- helping to shutdown excuses, helping to be the "weekend" at the end of a long work week, etc -- then that's a useful model to adopt.