This certainly could have been the case. An EPIRB that did not work would make a slow sinking scenario plausible. The only things I question in that case are what became of the liferaft? I assume they had one and why was no flotsam found? Maybe some will turn up but it is puzzling that in a full-scale search with a fair idea of position, nothing turned up. I don't know what kind of EPIRB they had but I am also assuming it was one which can be self-tested and that the captain did perform a recent test. This seems to have been a capable boat and crew, unlikely to have equipment neglect/failure of this nature.
We can only hope that the crew will be found floating in a liferaft with a malfunctioning EPIRB.