I've only recently read Strata, and I've heard it repeatedly mentioned that Strata not just inspired the Discworld setting, but the two universes are the same.
Is this proven/stated ever? Are there any clues that hint to/away from this?
In Strata itself, Kin insists that Jalo told her that he had found not merely a “flat world” but a “flat Earth”. (I can't find any reference to this in the actual dialogue between them). When the characters find and stumble on the world, they see that it is indeed a flat Earth. The geography, climate and other characteristics are those of Earth (the part of it that's on the disc anyway). This doesn't fit the Discworld.
As for the Discworld and the Strata universe being the same… I'll have to say no. The Discworld operates on narrative causality; this is a fundamental law of physics in that universe. The Strata universe obeys the physics that we're used to;
it's a creation disguised as a natural formation, which is an altogether different premise.
If you consider the Science of Discworld books canon, our own universe may have been created by an experiment at Unseen University. But the Discworld universe itself is a creation of our own universe. So while there is something to tie all these universes together, they live at different levels of creation.
In any case, the word of god is that they're different universes.
The main difference between Strata (and Dark Side Of The Sun) and the (later) Discworld series is that Strata is a parody of Larry Niven's Ringworld, and as such parodies common Science Fiction tropes. The Discworld, however, parodies common Fantasy tropes, and, as the series progressed, generic tropes in fiction alongside parodies of other institutions.
Even though there are some points of similarity (like the Discworld holiday of Hogswatch appearing in DSotS), the fact that Pratchett wrote his two Ringworld spoofs, stopped, and started writing the Discworld novels implies a "change of gears" into fantasy parodies, wherein he reused ideas and concepts from his earlier books.
According to the Annotated Pratchett File, a compendium of annotations, comments and explanations by Pratchett in the original alt.fan.pratchett newsgroup, the two do NOT share a universe:
Is Strata a Discworld novel or isn't it?
"Strata used the idea of a Discworld but I've never thought of it as a Discworld novel within the meaning of the act. The first Discworld novel was The Colour of Magic. Let the message go throughout the kingdom..."
This is backed up, though unsourced, by Strata's Wikipedia entry (emphases mine):
Although it takes place in a different fictional universe and is more science fiction than fantasy, it could be said to be a kind of precursor to the Discworld novels, as it also features a flat Earth similar to the Discworld. It has been called a "preconsideration" of Discworld,
The answer is yes... without a doubt...
The flat world is NOT discworld BUT as they say at the end f the book, they will have to create a NEW flatworld and the contentions will need to be shifted, new countries added, new creatures etc... e.g. the new world that is commissioned to be built at the end of Strata is diskworld.
Hence also the later links with things like the committee etc... when the computers and population are migrated form flatworld to diskworld.