From the linked article:
Of course, the patent for the RF lens indicates it's an extending zoom, with a nearly 70mm increase in length (that's longer than the barrel extension of the 70-300L). If the patent dimensions are accurate, at full extension the RF lens will actually be physically longer than the EF 70-200/2.8 III. Based on those patent dimensions, the sensor-to-front-element distances of the EF on a DSLR and the RF on a R will be very close, meaning the 'benefits of the smaller and lighter mirrorless system" are, in this case, pretty much non-existent.