This is difficult to say without better information for precisely how census data is gathered, how many people are using the addons for gathering the data, and how often they are online per server.
There was another thread discussing ConsensusPlusClassic, and whether it was broken when Blizzard changed the ability to automate /who() API calls from addons.
Basically, it seems that for concensus data to be gathered, people need to install the addon, manually start data collection, and upload the results. As the process is relatively convoluted, one might expect this to not be an addon for everyone.
However, with the populations we have on WoW Classic, is it a realistic assumption that there would be at least one character on either faction, on every server running consensus at any given time? We could certainly do some further investigation into how realistic that might be, if we choose to.
On the other hand, we also have data based on Warcraftlogs, which is representative of the numbers of raids who have at least 1 person in the raid logging and uploading to the warcraftlogs site. It seems reasonable to assume that this would be a good indication of the ratio of Alliance to Horde who are at least raid-ready and/or high or max level and/or raid geared or gearing. It also seems reasonable to assume that this would be a good indication of the relative differences in population size between realms, e.g. Faerlina has roughly double the raiding population of Kurinnaxx.
There are obvious deficiencies in the data we have available, making certain estimates or assumptions unreasonable, but some do seem quite reasonable to make.