There were not spell slots in the older editions, there were just spells.
The line between what's a tweak and what's a more drastic change is in everyone's eye.
I get what you are saying, but the term "spell slot" is an actual d&d term that is in the rulebooks that has a defined meaning. I believe the first official rulebooks that used the term "spell slot" is the 5E starter set and player's handbook.
Like you say, the rules are a tweak of earlier versions and the "spell slot" mechanic is most of what that tweak is. There are other minor tweaks as well, like short rest spell regeneration and prepared vs memorized spells, but the "spell slot" as defined in the PHB is the biggest tweak in spell casting from earlier editions.
