I find this part of the post a bit misleading. What does it have to do with privacy? The addresses you'll derive from that seed will be known from the server(s) you're connected to. The seed phrase doesn't provide privacy, it *just* recovers your wallet. Going non-custodially surely provides better privacy in contrast with an online wallet. But, keeping yourself private depends on how you will transact. (e.g: Making coinjoin transactions, revealing your master public key etc)

This is what you may wanted to say, but you haven't formulated it properly. You've added this under “Why should you extend a seed”, which isn't true. If you extend your seed you aren't increasing your privacy strength. You strengthen the security of your wallet. The odds for someone to successfully find a seed collision.