He actually didn't say it in those words. I've just binge watched all 3 now so only just saw the first episode. They talked generally about how people would be forming alliances as part of the show, Clare pointed out that a lot of people know each other already, Ruth said that put her and Nico at a disadvantage (used more colourful language), and Jonathan said "Well you've got two allies already here" referring to himself and Clare.
So I can understand why Clare backed Ruth up, as he did refer in a general way to being allies, but to me I think it's a subtly different interpretation - I understood him as using allies as basically a synonym for 'friends,' by acknowledging that it is hard for more unknown people to come into a group of more established celebrities, but he and Clare would make sure they weren't excluded. He clearly didn't consider it to be a formal offer of alliancing together as a group, which is why he probably didn't remember it.
It's a really interesting example of memory, how subtly different language can alter meaning, and how different people interpret words. It reminds of examples like Louise Woodward saying she 'popped' the baby on the bed, or Amanda Knox saying 'See you later,' to her boss, and the way those sentences were interpreted differently in the countries they were in.
I think it's ridiculous if Ruth genuinely believed at that point he was offering to form an alliance with them, as nobody had been picked as traitors/faithful yet.