As the 'someone' who "ran some tests on another Forum with MGIV", can I just say that those 'tests' were fairly non-scientific and undertaken purely at the request of another member of the Film & Darkroom User forum who was curious about the apparent conflict between what Ilford had previously stated and what appeared in reality.
Ilford are quite right to say that MGIV is not 'developer incorporated', that description should be reserved for papers like the old Ilfoprint material (of which I must have ordered tens of thousands of sheets in my time!). Some 'modern' papers do have a compound (or compounds) incorporated that will produce a degree of development in the presence of alkali, MGIV is not alone in this, and I would not expect any manufacturer to divulge what it might be.
It's worth adding that the Multigrade IV RC I used for the tests was some years old; how many I can't say exactly, but at least ten. The fact that it still develops perfectly under normal conditions, without any trace of fog or reduction in contrast, is testament to the quality of this product. It was always my paper of choice when working professionally and the stock I have left over is from those days. I now print only on fibre-based papers, so it has languished in a cupboard, at variable room temperature, for all the intervening years. Another testament to the excellence of this paper - long may it continue!