You can but no one will take those modified and increased supply coin, just like those forked coins.
So, let me ask you again. Can you technically increase the supply of coins? Even if its a hard fork? It seems like what you meant is that the rules of the network cannot be changed since the genesis block, while changes are still being made. It is possible with consensus.
Bitcoin diamond did it, they changed 21 million to 210 million coin. I think what he meant with "no one will take those modified coins" is that we only treat those forked coins as shitcoins.
Raising the supply is just a matter of changing the code, the hard part is to get everyone to accept that new coin.