We're gonna say the turret has 160AD (it starts at 152 and +4 every minute up to 250AD, but I'm gonna put it lower than it obviously was to make a point).
So: 160AD with 40% more damage every shot up to 120% bonus damage.
shot 1: 160
shot 1+2: 384
shot 1+2+3: 672
every shot after is going to be 288 damage so:
672 + 4(288) = 1824 damage with 7shots
This is assuming he started at full hp and didn't take any damage from his enemy laner that he was diving. We didn't calculate armour into the equation, but if we did then we'd have to calculate the tower's 30% armour penetration as well and that's a lot of work.
The point is, Fizz would be left with 200-300 hp after tanking 7 turret shots. Is that still a stupidly low amount of damage for a turret? Yes...but it definitely wouldn't have left Fizz with >1000hp after 7 turret shots.
Edit: And just as Azir Chroma pointed out, Fizz has 1224 health at lv.10. I didn't even bother to fact-check that part of your post but it turns out that was drastically exaggerated as well. Fizz would be 100% dead after 7 turret shots
Heres a tip: Keep the exaggeration down if you want a serious discussion about a topic (ie: turret damage). Exaggerate a lot if you are arguing a favorable mob-mentality point on the boards (ie: lee sin or yasuo hate threads) to farm upvotes.