The matrix tries to provide a nice transition to white for the brightest parts.
filmic tries to avoid colour shifts; but if I understand correctly, human vision has its own, which the AgX matrix tries to approximate (so you don’t get ‘salmon sunsets’).
The sigmoid module actually has the same matrix features (insetting/attenuation, rotation). Its source code and design documentation (PR) mention AgX as inspiration, see sigmoid: introduce custom primaries ("AgX-like" processing) by flannelhead · Pull Request #15104 · darktable-org/darktable · GitHub.
I think one difference is the curve, which in sigmoid can be quite flat at the ends (Naka-Rushton Tonescale | Desmos and Logistic function - Wikipedia, based on New module: Sigmoid display transform by jandren · Pull Request #12642 · darktable-org/darktable · GitHub mentioning the Log-Logistic / Naka-Rushton curve). In AgX: Power(P) Sigmoid | Desmos