"(...) The Linux 6.15 [ in the "Ubuntu MATE 25.04 ('Plucky Puffin') Release Notes" ] is referring to the kernel?
Indeed, in the "Ubuntu MATE 25.04 Release Notes", the expression "Linux 6.15" is referring to the version of the Linux kernel. As far as I can tell, @Wimpy (Martin Wimpress) usually refers to the Linux kernel version, in the Release Notes of the several versions of Ubuntu MATE, as "Linux [version]".
If so that should be 6.14, and maybe worth an edit.
Great catch! I've just submitted the following pull request to make that correction: