The LOR (local orthodox Rabbi) says that Slifkin is being challenged on three grounds:
1 - He denied the science of the sages. Example: He claims spontaneous generation isn't real.
2 - He denied the cosmology of the sages Example: He claims the universe is more than 5000 odd years.
3 - He denied the authority of the sages. Example: He says the sages are fallible.
According to the LOR, the first two are excusable, but the third puts Judaism on a slippery slope; therefore its intolerable.
But what about horayot*? Yeah, I asked that, too. Apparently, only the sages themselves can conclude that they themselves have made an error. Slifkin (a) isn't a sage, and (b) it's not his own error he's correcting. Ergo, his testimony is out.
This strikes me as foolish. Because if the sage dies before he realizes his mistake, what's our recourse? How do we keep errors from becoming part of the tradition, if no one can ever correct a sage?
Recipe for disaster, no?
*Tractate of Talmud that seems to recognize the fallibility of Sages.