I am not sure this refers to the mEHV mild hybrid, as the 48V battery stored under the passenger seat should have no impact on the space available in the boot for a spare tyre. But, perhaps it is rather the 17" wheels on the mEHV that makes the 18" spare unsuitable - this is exactly what I am trying to establish.
Interestingly, while on the Czech site they mention two exceptions (hybrid/CNG and sound system), on this site they only mention the sound system as preventing the use of this spare: https://shop.wingsskoda.co.uk/products/skoda-spare-wheel-set-octavia-iv