I'm also on Fi with a Pixel 3XL, and I have not received the update. This is the first time this has happened to me without a real explanation, and I've had the Nexus 6 (Dec 2015 - August 2016), Nexus 6p (August 2016 - August 2017), Pixel XL (from August to October of 2017), Pixel 2XL (Oct 2017 - Sept 2019), and now the 3XL (Sept 2019 - current).
I've been on Fi since February of 2018. Before then, I was on Verizon. I also have a Pixel 3a on Fi, and it received the update last week (which was also late).
Perhaps I'll wait until next month and see what happens.