[sophacles]

It is their doing - they started the series of events. It's other people's doings ALSO. This sentence doesn't imply exclusivity, it just doesn't say others did stuff. Fortunately the entire rest of the document, particularly the part I highlighted above, makes it very clear:

1. what their doing is in more specific detail

2. what others' doings are in more specific detail

I'm going to assume that the author included all that additional detail in the document on purpose, and with the intent of clarifying what "That's my doing" entails. It seems pretty unlikely that your hypothesis of "the person is claiming the whole thing is their doing and accidentally wrote a whole bunch of words undermining that simple sentence" is the right one.