Segwit is slightly different, it was to strip additional metadata that would need to be written to the block from memory... HD is the ability to derive many public/private key pairs from an initial seed across multiple Bitcoin-derived chains. It is what makes Ledger able to handle many Bitcoin based chains easily without 3rd party applications or support. The onboard address manager handles all the intra and extra transfers seamlessly to make it appears like you only have a single wallet instead of many millions/trillions of linked one-use addresses!
PS: Apologies if you already knew this!
I remember people calling the wallets segwit wallets, there were 2 or 3 BIP involved with the HD wallets. I don't think those even required hard forks. The main segwit update was huge and unrelated as you mention. It is all confusing. I really need to get back into studying other blockchains, especially before I invest in new ones.