Saying it’s a forgery is too much. It has been clearly subject to a process of evolution, and that process has been more extensive than for most suttas. Crucially though, it is the latest and most inauthentic sections that are most emphasized in modern times. Given the scale of development, the evolution of the text persisted well into sectarian times, hence I would place it in the very latest strata of texts in the Pali nikayas.
Moreover, it is the only substantial sutta that has been subject to major editing revisions in modern times. The history is not yet established, but by the 6th, and even it seems the 5th counil in the late 19th century, MN 10 Satipatthana had been expanded with all the additional material on the 4 noble truths from DN 22 Mahasatipatthana. My guess is that this was a 19th century Burmese innovation.
When restored to the likely original core, the most noteworthy thing is that the content that explicitly deals with vipassana disappears: i.e. contemplating rise and fall, etc. That is, except for the 4th satipatthana, which is where vipassana finds its original home in this teaching. Satipatthana is primarily samatha; i.e. the basic purpose of it is to get into jhana. This is not an inference: it is stated explicitly in the suttas. (MN 44: "the four satipatthanas are the basis for samadhi = cattāro satipaṭṭhānā samādhinimittā)