Looks like misplaced caution.
If you aren't absolutely sure what you're doing, a ground plane for all nodes is almost always better than trying to hack it otherwise.
Yes, routing those traces together will tend to reduce EMI, but has that principle been applied consistently? One tiny buzzer at 2kHz seems a preposterously low priority considering the likely noisy digital logic sprawled out elsewhere.
Consistency is another one of those things that bugs me. Lots of average designs are inconsistent, sloppy, careless.
Using such examples as reference obviously doesn't make for good learning opportunities..
The entry angle of the trace into the plane seems to suggest it was routed before pouring, and the pour was made later as an afterthought. Possible it was even a revision hack, done in the laziest way possible.
There are no thermals on the pour, and the pour is bottom side only. This is bad for fabrication because the copper imbalance causes the board to warp slightly. Maybe not a big deal for QFPs, but a possible problem for QFNs, LGAs, CSPs, etc. Lack of thermals won't matter much for an apparently wave soldered board, but does seem to show lack of consideration.
Tim