I agree it is plausible that the flat-bottomed crater is due to infill. However, since the report is that this is a region of hydrovolcanic activity we should *expect* that there would be maars, tuff rings or tuff cones in the region.
However, there would of course also be impact craters as well in the region as there is everywhere on Mars except for the very youngest regions.
Impact craters can be difficult for even trained obsevers to distinguish from maars, or other volcanic craters from orbital imagery alone. The determination is usually made on the ground from analysis of the rock in the area.
Also note that this report I cited previously that argues that some craters seen are actually maars rather than impact craters:

Evidence for volcanism associated with Ismeniae Fossae fretted channels.
Workshop on Early Mars (1997)
http://www.lpi.usra.edu/meetings/earlymars/pdf/3003.pdf

also concerns a region on Mars with abundant fossae, as is this region of Cerberus Fossae in this latest report. Fossae means fissures and both Cerberus and Ismeniae have often be suggested to have been regions of hydrovolcanic activity. The Ismeniae Fossae report was based on Viking imagery. Further MGS imaging may further support the idea that some of the craters seen in the area are hydrovolcanic in nature.

This latest report mentioned craters in association with mesas. If some of the craters are at the center of mesas that would support the idea that these are volcanic cones rather than impact craters.

Bob Clark