But atom is made by github, right? Not to justify their choice but you can see from a company perspective how it surely aligns with their goals (increase github market share).

I wonder what the market share is and if they're actively seeking it at this point.

I wouldn't say that they necessarily want to increase their market share at this point (but that may have been the original intent). Now it is probably more about retaining as many users as possible by offering more or more convenient services than their competitors.