Lightning can infinitely divide each BTC, but the Bitcoin blockchain cannot without a hardfork.

For one reason: BTCs are not divisible by sats at all. In code, there are only sats.

In the following file, line 15 shows sats (100mil); and line 26 shows 100mil multiplied by 21mil: https://github.com/bitcoin/bitcoin/blob/master/src/consensus/amount.h

The supply of Bitcoin is not 21mil BTC divisible by 100mil sats. The supply is actually 2,100 trillion sats. That's it.

This isn't splitting hairs. There is no division going on anywhere in that file. To increase the number on line 15 (sats per BTC), you wouldn't be increasing the "divisibility" of Bitcoin. You would be increasing the supply of Bitcoin (2,100 trillion sats). And that would require a hard fork.