My personal guess is that one way or another, Tuuri gave herself away when she claimed that Reynir was hearing the voices for the first time. The comic has yet to be clear about whether Reynir can hear trolls, but his outburst towards Lalli and the following conversation happened because he could hear Sleipnope also. Basically, Lalli probably has a good idea of where Reynir stands in terms of "magic hearing" capacity, and Tuuri's claims are most likely not adding up with that.