Boots are not a guarantee that you will be able to stay airborne when icing is encountered. The FIKI cert requirements are specific and do not include SLD or severe icing. Some airplanes by their basic design are going to handle ice better than others. The Aztec handled it well because the thing was so unaerodynamic it was hard to make it worse. Twin Cessnas do ok, but not as well as the Pipers. A plane with more antennas will do worse because the antennas will collect more ice and slow the plane down further, causing it to collect even more ice.
The effectiveness of boots is also not fantastic nor is their reliability, and we don't know the condition of the boots. If they had holes in them then they might not have worked well, there could've been a leak in the system, some bad valve, or they could have been left untreated. You need to treat them in order for them to have maximum effectiveness. The 303 is underpowered, and they had 4 people on board so it was somewhat heavy. Not something I'd want to take into icing.
Judging from the damage, it looks to me as though they ended up stalling at some point off the ground rather than making a fully controlled landing. Fortunately that looks to have happened close to the ground, or else they would've ended up with much more than minor injuries.