Hmmm… most MEMS microphones (but not all) output Digital, not Analog and need connections to GPIO0 and 1 (which also requires Device tree changes). Based on the fact that your microphone wants 3.3V, I would suspect it is a digital MEMs microphone.
The diagram you have attached doesn’t show the full circuit, you haven’t considered the contribution of the microphone in the circuit. Although you are correct the bias supply voltage is 1.8V, this is not the voltage that will be on the pin during operation.
A typical analog microphone has a DC resistance of about 2k Ohms. The internal bias resistors shown in the diagram are “pull up” resistors with values of about 2k Ohms. When you have the 2k Ohm resistor pulling up to the 1.8V Bias supply, and the 2k Ohm microphone pulling down to GND, you end up with a effective offset voltage of 0.9V at the input.
When you are connecting an external line input, you don’t want to use the internal 2k pull-up resistor, since this will do two things you don’t want. 1) it will give you and input impedance of 2k Ohms which is non-standard, and 2) will leave with a 1.8V DC offset, which is incorrect, you want a 0.9V offset at the inputs. Hence two 100k resistors and a capacitor…
You will need to ensure that the internal bias supplies are disconnected when you are using the inputs as line inputs.
FYI - the diagram you attached contains the note “Capless Input only” under each ADC, this is the (hidden) answer to you original question about AC coupling capacitors.
Full disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other posting may not reflect the opinions of my employer.