It's not the dual slot that breaks TWRP. Our phones had dual slots even under Android 9 and TWRP worked just fine. The Pixel 3a and 3a XL are also listed as supported on the TWRP website.
The issue is that the disk/file structure is vastly different between Android 9 and Android 10 phones. Originally TWRP only worked with Android 9 and it's disk/file structure. The TWRP team was able to work some magic and now TWRP 3.4 will work with Android 10 as long as the phone itself has the old disk/file structure from Android 9. A phone that was updated from Android 9 to Android 10 via an OTA update should retain the old disk/file structure and TWRP should work.
The "problem" is that only "enthusiasts" like us care about TWRP and it's likely that most enthusiasts have flashed a full factory image at least once since upgrading to Android 10. I am guessing that flashing a factory image changes the disk/file structure to the default Android 10 structure and this is why people cannot get TWRP to work on these phones. However someone that upgraded to Android 10 via the OTA image that hasn't subsequently flashed a full factory image may be able to get TWRP 3.4 to work with our phones.