I can't find this in the FAQs so I think someone gave you bad intel.
Page 8 of the Core Rule Errata says: Warscroll battalions that share the same allegiance as an army can always be taken as part of the army, and if they include any allied units, these units do not count against the limits on the number of allies the army can have (or against the points limit that can be spent on allies in a Pitched Battle). An army can include a warscroll battalion of a different allegiance to the rest of the army, but if it does so the units in it do count against the limits on the number of allies the army can have (and the points for the battalion and the units in it count against...
I'm still a bit confused too. We should all probably start quoting sources to avoid it.