On Linux, the GUI should have a key-export function. It may be in the advanced settings, and be called “Export private keys”, though it will probably export the public ones too. It tells you the export location.
The Android GUI key export options were removed due to an Android-specific security problem, whence the post you linked. Since the security problem was not present on Linux, the functionality is still there.
My DeltaTouch DBs are in the subfolders of ~/.config/deltatouch/accounts (almost). There is one subfolder and one “.db” file per account. I know you are not running DeltaTouch but it should be similar.
If you just want your public keys, open the “Saved messages” chat, press the attach-button (“+”)>Contact>Me. Send, then export the attachment to an “.asc” file. It will contain your public keys.