What DSP method are consumer products like the game Rocksmith and guitar polyphonic tuners likely to use for multiple f0 detection? The game Rocksmith and some polyphonic guitar tuners (like the PolyTune) are able to recognize, at least to some extent, multiple fundamental frequencies in single-source audio.

I know there are some methods for multiple fundamental frequency detection, like MUSIC and ESPRIT, but is it likely that these products implement them or leverage several assumptions (such as: audio is from a guitar, audio is from a single source) about the application domain to make a simpler method work?

In the second case, which simpler methods are likely to be used? Simple peak-picking on a spectrum, with domain-specific heuristics? Or something more complex?
Check out chapter 1.3 of this IRCAM paper on multi-F0 estimation. It discusses the difficulties in extracting multiple F0s from a recording, including the handling of overlapping partials, transients, and reverberation, as well as the modeling of domain-specific sources with varied spectral properties.