Yes, the 3140 will handle that just fine. configure a SIP trunk to Comcast and one to each of your PBXs. I'm not sure if the link you provided was supposed to be a diagram or related to this configuration, but in reality it was a random advertisement.
I'm not sure what you mean by emulation mode. The 3140 doesn't have a way do directly provide voice services such as analog lines or PRI. You can use the TA900 series to emulate a PBX. For that matter, if your stations are analog, you can use a TA900 as a PBX.
I think what they mean is that the Netvanta side connecting to them would work the same way a PBX does and yes it does. Basically it would mean it is setup as a Back to Back User Agent. The 3140 registers to the Comcast Sip Trunk, and then the phones register to the 3140 SBC and the SBC handles the call flow.