I think your interpretation is right. You will achieve the full resolving power of the sigma even in crop mode. Which is, I will not hesitate to admit: surprising to me!

However, it's worth noting a few things:

• The full frame device ultimately has a higher resolution by up to 33%.
• It is also about a stop faster.
• If you have any notion of using the lens for medium telephoto, you can forget about it with the Sigma (minimum ff equiv focal length = 150mm vs 70mm with the Sony).
• But you have better super telephoto capabilities with the Sigma.

Ultimately I think there is a place for both of these lenses. But I'm really surprised about the MTF numbers, because I would have sworn there was an advantage to having 33mpix. Unless these are both very poor lenses, this suggests that the real advantage of higher megapixels is nil.