That's straight away misinformation. Your body doesn't know how many plates your lifting, all it cares about is intensity and time under tension.
There's a few ways to increase that without increasing weight:
- more time under tension (duh), in other words same movements but slower
- harder exercice variations
Now to mention that kettlebells can go pretty high in weight.
And a last point: what is your goal? Kettlebells will never allow you to compete in bodybuilding, but if what you want is to have a better quality of life and aesthetics? The answer is much less cut and dry. Kettlebells have some really great functional exercices. If you can do a turkish get-up with a 50kg kettlebell you're strong enough.