Android and postmarketos are both linux in the same way as whales and rabbits are both mammals. It's true, but not usually the useful way to compare them.
Do remember that this device is running a full kernel, so actually you can get the speed and memory benefits of running such. Expect some Raspberry Pi 3-eque performance.
The biggest bottleneck in RPi performance has always been the slow SD card access times. I have hardware from the Pi 3 era that still flies because they run off SSDs.
They definitely seem low when compared to androids, but If 2 GB works as great as it does on my iphone 7 then it should be doable to get a good experience here too. 3 GB is the max ram for the chip, hopefully they come out with a version that has that, or one with a different soc that can support more.
Like the other guy said with SD card you won't have an issue with storage.