This means that while they offer both a standard SIM slot and an eSIM, they can't both can't be activated at the same time, meaning users can only talk, text or use data on one line while the other remained largely useless.
This information comes from a comment left by one Google developer in Android's code documentation, so it seems reasonable that it is legitimate. Crucially, the commit says it can do this "even if [the device has] two or more SIM cards". Just as you would on a phone with Dual SIM card support.
While bringing DSSS support would require Google to add a second antenna which is not possible for Pixel 2/3 but Google might bring Dual SIM Dual Standby (DSDS) to older Pixel devices.
That's going to change with the 2019's Pixel 4, though, which will likely adopt Dual Sim, Dual Standby (DSDS) tech. So, the minute that you switch over to the eSIM, you're no longer able to make or receive phone calls from the number assigned to the physical SIM in your smartphone.
Before then we might well get treated to the Pixel 3 Lite - Google is rumored to be readying a mid-range version of its 2018 flagship phone, built with slightly cheaper materials and a less powerful chipset on board. You aren't required to purchase two identical phones to take advantage of the promotion, any combination of Pixel 3 or Pixel 3 XL phones will work regardless of their storage capacity.
Whether this means the Pixel 4 will support two hardware SIMs (as most users would understand dual SIM to mean) remains to be seen, but it appears if it does, the software will be ready for it.
Beyond that, there's no other word regarding other features. Interestingly, the same commit stated that the dual SIM capability can be enabled on Pixel 3 devices, even though it's only for dogfooding.