I didn't say idoneth need a new battletome. I said that given we have a rumour pic that relates to idoneth and may be their endless spells there is a possibility of a new book. This is primarily based on an assumption that GW don't want to release endless spells on their own without anything else to support the release.
That said I do believe that idoneth could benefit from a new book, as some of their models have lacklustre rules. A lot of the mechanics designed to support thralls and reavers don't work well and there aren't many battalions.
At the moment the cracks in the army are papered over by the strength of morsarr guard on the charge.
However, this may be something that is fixed in GH19.
Regardless, I expect something to be released alongside endless spells whether that be a new hero, a battle box or a book.