Since the crater clearly pre-existed the fluid flows (based on superposition and crosscutting relationships) that breached its rim, the flat bottom could be nothing more than sedimentary infill from the primary flood(s) and/or during its waning stage. The crater is highly eroded in any event, and postulating that it represents a maar or a phreatic cone based on superificial morphology analysis is speculative at best. At any rate, at least one of the authors (McEwen) of the paper:

Burr, Devon M.; McEwen, Alfred S.; H. Sakimoto, Susan E.
Recent aqueous floods from the Cerberus Fossae, Mars. Geophys. Res. Lett., 10.1029/2001GL013345
15 January 2002

is a renowned vulcanologist and I presume he would recognize a maar, even a highly degraded one, when he saw it. In fact, McEwen and colleagues *have* postulated the existence of phreatic cones elsewhere on the Martian surface. See

http://www.psrd.hawaii.edu/June01/lavaIceMars.html

Alex R. Blackwell
University of Hawaii