Edit: from Wikipedia's Delta-sigma modulation it looks like since the microphone basically is an ADC, and that as such mics with PDM output may use a Σ‑Δ modulator internally. Also, from Selecting PDM Microphone Clock Frequencies and Decimation Ratios I read that "A Pulse Density Modulation (PDM) microphone uses a Sigma-Delta modulator to oversample an acoustic signal at a high sampling rate. This digital PDM signal is output..". Adding from Marcus' comment, maybe the "smart" is marketing only(?)
I see that there is something called "Smart MEMS microphones" which contain a Σ‑Δ modulator. Like T5828, T5837, T5838, T5848 from TDK InvenSense (SmartSound™ Overview).
Assuming that older MEMS mics with PDM output would not contain this, am I right in assuming that it is the addition of the Σ‑Δ modulator that makes it "smart"? Or would this be along another axis? Like level detection.
These mics still present a 1-bit datastream that must be decimated in SW.
What would a Σ‑Δ modulator in the end add?