In the interview, Takashi Ueno of Fujifilm explains why he thinks the XF lens mount is inadequate for in-body IS - shifting the sensor means some parts of the sensor will be shaded by the mount. In other words, the ideal lens mount should be larger diameter.

Now, is the image quality compromised in the recent Fuji X-H1, featuring IBIS? According to the above statement - yes, it is compromised. But I doubt the difference is easily noticeable. It is just a little bit more vignetting. After all, and we get some vignetting all the time, regardless of the image stabilization. Fujifilm's small APS-C sensor enjoys plenty of space behind their XF mount, compared to the Sony full frame cameras using only 2 mm wider Sony E-mount.

Alternatively, we can guess that the Fuji guy simply wanted to turn the camera's deficiency into merit. Look, our competitors offer IS, but we don't and this is good, because it means better quality. Now, 2 years later, let's forget about it, IS is now good because we have it as well.