Electronic – For an inverting opamp, is inverting the same thing as an 180 degree phase shift

The phase margin PM is a measure for the stability of a system with feedback. And, thus, it also applies to operational amplifiers. The PM is defined for the LOOP GAIN of the system - that means: open the loop at a suitable node and measure/simulate the gain and the phase around the complete loop. Then ,the PM is the DIFFERENCE between the measured phase and -360 deg (that means: The "distance" to the oscillation condition, positive feedback) at the frequency which gives unity loop gain. Without taking the phase inversion at the inverting input into account, the PM is the "distance" to -180 deg.

Now, for an opamp the most critical situation arises for 100% feedback (unity gain operation). In this case, the feedback factor is unity and the Loop gain is identical to the open-loop gain Ao of the opamp. Normally, only this condition is used to specify the PM in the opamp´s data sheet.

Summary: The PM as given for an opamp is the DIFFERENCE between the opamp´s phase shift and -180 deg at the unity-gain frequency.

For your frequency sweep at frequencies much lower than the gain-bandwidth of the amplifiers divided by the gain (10) the magnitude will be a constant gain of 100 and the phase will be 0 degrees. (Or 360 if you prefer.) The time delay is negligible at the lower frequencies so you won't see it.

However, at higher frequencies you will see the contribution of both amplifiers to the rolloff and phase shift, which will be different from a single amplifier with another 180 degree phase shift. So they don't really cancel each other out except at low frequencies.

So for low frequencies gain is constant and phase is pretty much the same as the input signal. Why would you expect something different?