It's def not coming anytime soon. They're a super small team and Silksong is supposed to be bigger and have all new enemy types and content.
HK was announced and kickstarted in 2014 with a beta in 2015.
Game actually came out in Feb 2017.
And they've been working on Silksong ever since with all that experience under their belt, and with some of the concepting and such being done even before HK's release considering this was originally gonna be a DLC for it. One of the areas in Silksong is even a scrapped area from Hollow Knight that they probably salvaged a lot from.
Idk, I understand that Team Cherry is a very small team but you'd have thought that HK and its free dlc releases would take longer than they did from that info alone. They work extremely fast for such a small team, and when they say they already have more enemies and music made than HK launched with, it doesn't sound like it still has over a year left to go, especially with how polished the E3 demo was. I'll be fine with it if it really does take that long, of course, but I'm optimistic for a Summer or early Fall release.