The 6 month rule after I-485 filing is a black and white rule written into law. The 6 month "rule of thumb" for staying after green card approval is a different thing and is just a rule of thumb.
However, the green card is for future employment, and by filing the I-485 you are declaring your intent to work for the sponsoring employer after the green card, for a "permanent" length of time. Leaving too soon (whatever in the world is the definition of "too soon") after green card approval is an indicator that the intent to work permanently was not there.
Of course, in an employment context nobody reasonably interprets "permanent" to mean lifelong. I think the 6 month "rule of thumb" is derived from the 6 months (actually 180 days, to be exact) of the actual AC21 rule .... by passing AC21 into law, Congress has acknowledged that employment intentions can change in 6 months without fraud or misrepresentation. So if it is legitimate to leave the employer after 180 days since filing the I-485, it would not make sense for something to be wrong with leaving after 180 days since I-485 approval.
But if 180 days post-approval is safe, what about 150? 120? 90? And is 180 days post-approval really safe anyway? Until there is a court case or regulation to clarify it, we won't know when is the exact point that changing jobs post-approval becomes safe. So we just have to evaluate our own situation and decide the balance of risk vs. reward when we take action (or inaction).