There is reason to disbelieve it. The text analyses that have been done point at Satoshi being an individual.
Beyond that, if it were a group, they could have used someone as a spokesperson, but then you have a conspiracy of Satoshis, which is less possible. People, and especially geeks, like to talk. Someone would have tripped up by now, or moved bitcoin to fiat.
I agree that the group increases the risk of someone talking.
Would having a spokesperson that knew the identity of group members be a significant risk once this amount of money is involved? It would surely motivate serious threats to the spokesperson and their family?
By the time there was media interest in the identity of Satoshi, what would have been the value of their holding? Even then, whoever it is had to have held to their privacy even in advance of Bitcoin being worth incredible amounts of money.
Beyond that, if it were a group, they could have used someone as a spokesperson, but then you have a conspiracy of Satoshis, which is less possible. People, and especially geeks, like to talk. Someone would have tripped up by now, or moved bitcoin to fiat.