Just because someone works in the field, doesn't mean they're always going to be right. A lot of old-fashioned advice that has been disproved or revised in recent years is still doing the rounds in some aquarium circles.
Best practice dictates that fish should, almost from the very start, only be kept in tanks that are suitable for that species of fish for it's entire lifespan. Fish in tanks that are too small suffer numerous health issues, including physical deformities, pathological behaviours and a significantly shortened lifespan.
It is true that you cannot stock, properly, any tank just from numbers of fish and volume of tank. There are many, many other factors that must be taken into account, if the fish are to live 'happy' (as far as well can tell) and healthy lives.