That's a misleading statement with no valid proof to back it up.
Your phone's charging circuitry will take in the voltage and amperage it requires so using an adapter that can supply more power isn't going to be a problem. Power generation and transfer does produce heat, the issue being when using an adapter that's too minimal it will always be heating up much more. It's being required to work harder at a more continuous rate. On the other hand, using a power adapter that's more capable won't be struggling to provide enough power, and the charging process is going to be better for the battery too. Most power adapters are more or less cheap and incidental anyway, being too small to contain really effective filtering components to provide clean, precise power delivery. And for most people they'd rather replace a failing power adapter than a failing battery. Power adapters are relatively inexpensive and easy, while replacing the battery in a lot of smartphones now is a hassle.
The Battery University site has a lot of good info that gets updated regularly if you want to bone up on factual info about battery care, charging, storage, etc.
https://batteryuniversity.com/learn/article/all_about_chargers
https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
https://batteryuniversity.com/learn/