Likely so.
The goal is to be able to compile identical "reproducible" binaries later, so the file times are adjusted in a definite way (to the time of the last git commit). Likely also the kernel version time is set the same way.
EDIT:
Most likely the behaviour is due to
https://git.lede-project.org/?p=source.git;a=commitdiff;h=be6acba4dc5c9013a8f61fb2e9d2dbb976126aa8;hp=ad8169546cbcb5b2ed59cef332bb6b1dc5e389ef
and uses the time set by
https://git.lede-project.org/?p=source.git;a=commit;h=82522dbaee8cd5f94267ebf465e36d594e4d2d7a