I'm in the same boat. I got a HomePod on Saturday and it was on 11.2.5, said it was up to date. After trying basically everything I have found to get it to update it found the 13.3 update last night about 6:00 pm.
Now a new set of issues.... Last night I tried so many things to get the download to complete, from power cycles, router power cycles, removing it from my account, resetting it, three different iSO devices. I left it paired with my 11 inch iPad Pro overnight and it's still shows updating on the HomePod tile on the main screen of my Home app. If I go into Software Update section it starts checking for update finds the 13.3 and "starts" downloading it and never completes.
Edit: Now that 13.3.1 was just released I updated my 11 Pro Max and 11 inch iPad Pro removed the HomePod from my account, reset it. It found the 13.3.1 update and downloaded it within 10 minutes. It's installing now.