"non-bind mount source /swapfile is a directory or regular file" warning
Solution 1:
The linked article doesn't say anything about using findmnt - what output do you expect from it and why?
If sudo swapon -s shows your swapfile then everything is all good and the instructions worked.
Solution 2:
While using findmnt --verify will test the fstab before rebooting (an incorrect fstab can render a disk unbootable).
The error for swap file is a common warning and needs no attention.