I wasn’t aware that there is so much confusion about the meaning of “mainline”. I suggest reading https://wiki.postmarketos.org/wiki/Mainlining (this is the second time I’m posting this link here).
Most hardware manufacturers do not use mainline kernels. They fork the Linux kernel in a way that will never be accepted upstream, adding custom drivers that only work with Android blobs.
A mainline kernel is the opposite: drivers are generic and should work with standard Linux distributions. Any differences to the upstream Linux kernel releases should stay minimal. Devices with mainline kernels should not require any blobs besides actual firmware that runs on separate processors.
All official Sailfish OS ports use the kernel provided by the manufacturer, which is why they rely on Android blobs. Sailfish OS is technically capable of running natively with a mainline kernel. However, this is not very likely to happen on the new Jolla phone out of the box.
It is still possible as a community effort, similar to Mainline Linux kernel for the Jolla C2, but porting drivers from scratch usually takes a long time.
BTW, postmarketOS is constantly improving.