Given that Varis is even more passionate about dealing with Primals/Eikons than Solus I can't help but suspect that there is much more to it all - a piece of the puzzle that we do not yet know. Varis seems to be convinced that Hydaelyn as a whole - which he refers to as a 'Star' - suffers immensely and risks being snuffed out as a result of the summoning process. He isn't wrong, either. Yet he seems to know a lot of the same things that the Warrior of Light has come to know. A certain shadowy figure cannot be wholly responsible for that knowledge, either, as it appears as though Varis has only been dealing with him recently due to the lack of trust and familiarity. My guess is that a lot of the information came about through Allagan records.