Is it true that only Jehovah's Witnesses can state with certainty that they all agree in every doctrine?

Yes they can! Even when they don't as seen in the difference in the resurrection hope for those in Sodom and Gomorrah. The Revelation Book said the opposite with the Insite in the scriptures even though they were printed only months apart but since they are a uniform in their teachings, the Insite book was corrected to agree with that the WTBTS governing body said through new light.