[Klaus]

If the Olympus 100-400 is really a Sigma design (as I've heard loads of times already) does that mean it's an FF lens in disguise?

Yes.  The design schema is identical to this one:

https://www.sigma-global.com/en/lenses/c..._400_5_63/

The dimensions and weight also match - plus-minus a little due to the "extended" tube and Oly styling.

FWIW, it's not the first outsourced lens - the Olympus 75mm f/1.8 is also based on a Sigma patent it seems.

Cooperations between manufacturers aren't unusual anyway. Think of Tamron/Tokina & Pentax.

And Tamron has been "known" to do designs for Sony I think. There are also rumors that the Batis lenses are made by Tamron.