He would have found only
281 hits for "BM_CLICK."
This one might be of interest:
http://www.minigui.com/api_ref/group__ctrl__button__msgs.html
and the part where it states:
Define Documentation
#define
BM_CLICK 0xF0F5
Simulates the user clicking a button.
An application sends a BM_CLICK message to simulate the user clicking a button.
BM_CLICK
wParam = 0;
lParam = 0;
Definition at line 822 of file control.h.
That seems to make the "definition" of BM_CLICK and WM_CLICK seem somewhat similar.

At least in this case.
Regards,