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That shape has three cusp-like corners -- one at each sharp point, with smooth curves (convex/concave) between them. This satisfies the "at least two" condition.

The even simpler "lemon" is an even more egregious example, as it satisfies the conditions in two and three dimensions and yet is rather old and well-defined: https://mathworld.wolfram.com/LemonSurface.html

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I think that tile shape has one cusp-like corner and two ~90-degree corners. Thus it's not a complete transformation of a polygon into a soft cell, and hence the "at least two" rule doesn't apply.

They aren't 90° corners -- and in any case it would be simple to modify the angle of those corners and keep the shape basically (or nearly) the same.

Also the plain 2D lemon/lozenge/lentil satisfies all conditions.

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