I'd never heard that before, but you're right -- "Coke contains 32 mg of caffeine per 12-ounce (335-ml) serving. Diet Coke is higher in caffeine, with about 42 mg per 12 ounces (335 ml)." [1]
I wonder why that is? If people expect an energy boost from Coke, and if sugar isn't contributing to that any more, a little extra caffeine will?
Coke Zero, on the other hand, "contains only 34mg of caffeine per can (12oz)" [2]. So it's more like regular Coke.
[1] https://www.healthline.com/nutrition/caffeine-in-coke
[2] https://lifeboostcoffee.com/blogs/lifeboost/caffeine-in-coke...