That is not true, most of what defined them was the same. According to the rules, Paladins and Rangers even became regular fighters with an alignment shift.
In UA, a Cavalier can become a regular Fighter (but with weapons of choice rather than weapon specialisation) under certain conditions. Yet - for reasons that aren't clear to me - the Cavalier is not flagged as a sub-class of Fighter.
What defines a Fighter, in AD&D, is an attack and save table, a d10 HD, multiple attacks, the ability to use any armour and weapons, and the lack of an alignment restriction. A Fighter who builds a castle gets benefits.
What defines a Ranger, in AD&D, is sharing the fighter attack and save table, having a d8 HD (2d8 at 1st level), multiple attacks that are different from the Fighter's, the ability to use any armour and weapons, multiple special abilities like tracking and surprise enhancement, and having to be of good alignment and adhere to certain limits on owning wealth/goods. A Ranger who builds a castle doesn't get the benefits a fighter does.
Being a sub-class of Fighter dictates a ranger's attack and save tables. That's it. It doesn't dictate ability score requirements, nor HD, nor special abilities, nor alignment and related requirements. It doesn't dictate magical item use (qv Illusionists) nor weapon and armour use (qv Clerics vs Druids).
It's nothing like the concept of sub-class in 5e. In fact the only thing that sub-class defines in AD&D is irrelevant in 5e, where everyone uses the same progression table (ie proficiencies) for attacks and saves.