Here's an overview of phones with the dual-frequency chips:
https://medium.com/@sjbarbeau/dual-frequency-gnss-on-android-devices-152b8826e1c
It's hard to find good info and nothing with respect to Collector. People using gps testing apps were still getting 4m at best.
The pixel 4 was just announced with this note:
⁸Dual Band (L1+L5) or (E1 +E5a). Support - coming soon
So maybe the phones right now don't have proper software support?
I think there is still hope for improved accuracy from our cell phones with the dual frequency chips. I found an article where they tested the Xiaomi Mi8. They were able to achieve decimeter accuracy through post processing and sub-meter real time. Looks like the software is behind the curve, hopefully that will change. Here is a link to the article, it is a pdf.