Yes it was his NIP.  @Vitor Pamplona and @hodlbod and with NIP-44 @paulmillr as well as some other people providing lesser contributions.  NIP-44 was audited, but NIP-17 wasn't.

I thought it was a great idea and put initial GiftWrap support into gossip very early (9 months ago).

But NIP-17 DMs don't do everything SimpleX does.  Neither is strictly better than the other.

Still I think NIP-17 DMs are good enough. They use ephemeral keypairs so just like SimpleX it is like there are no IDs. And I don't think forward secrecy is really worth it.