It is the default exponential envelope shape (or VCA response), where both the envelope rise and fall are concave up.
My XM-OP does not have the exponential option, but rather has an envelope curve control that gives the more typical rise concave down and fall concave up. But I agree the exponential response sounds great. I think if you don’t assign the XM-OP envelope to the level, but rather patch the envelope output to a VCA with an exponential response, and then send that output to the XM-OP level you can get XM-OP to sound similar to FM-OP. I have recently been thinking about an XM-OP enhancement that could give the exponential response. I’ll put a post in my Venom development thread about this.