Overexposure is a less serious error than underexposure. Overdevelopment is a less serious error than than underdevelopment. If you claim not to know this from experience, shame on you for lying to your elder.
It's pretty obvious that detail that slides off the bottom is gone for ever. At the other end, a print made of an underdeveloped negative on grade 3 or even 4 paper is usually better than a print of the same scene made from an overdeveloped negative on grade 1.
That rule is so old that it may have been inspired by the use of printing out paper, which loves overdeveloped negatives.
Part of my photographic education has been learning how much leeway there is in these technicalities. I'm sorry to say that it all boils down to that first paragraph. David Vestal puts it "Expose generously. Don't develop too much. Make mistakes to find out how much is too little exposure and too much development." That's not an exact quote, but it's close enough for government work as we used to say at NACA.