@winni
I think we are talking about apples and oranges.
The figure I quoted arises from counting += in only 3 kinds of fpcsrc files: .pas, .inc, .pp
You are counting in files such as .fpc, .fpcmake, makefile*, .msg, .dat, .js, .ppu, .it, .png, .tga, .res ,.pdf and others too it seems.
These are all beside the point. My point is solely about Pascal source files in the fpc source tree, which abound in their use of +=.
I am not trying to promote the use of Cisms. I merely point out that they were introduced by an FPC developer who clearly convinced the rest of the core team of the value of this abbreviated syntax as an extension to traditional Pascal.
And either he or other developers make free use of the syntax now it is there.