I think this is the wrong argument to make here. Even if it would require a hard fork, such a hard fork is not impossible. If in 100 years bitcoin is being used around the world and is worth $10 million per coin, then we can't have the scenario of the smallest spendable amount being ~$20, and so a hard fork to add more decimals may well take place. Further, Lightning network already goes to 11 decimal places and milli-satoshi, so already provides a 1000x increase in divisibility over the main chain. However, neither of these things change the fact that there a finite number of bitcoin which can ever exist.
Well OP wasn't arguing about what may or may not happen in the future but what is true right now (which obviously is false).
As for Lightning Network, what the second layer does is not going to change the bitcoin protocol. So bitcoin is still not divisible to any smaller units than a satoshi. Another second layer could give you 1 bitcoin for every 2 bitcoin you lock in a channel and that still wouldn't change anything about bitcoin protocol.