I just ordered an evic today and I can't claim to be any sort of expert on these kinds of batteries as I've only ever used ego batteries but I know that most electronics with rechargeable batteries advise to avoid prolonged exposure to temperatures above 120 F. Whether this holds true for 18650s I don't know, but 98 F seems a bit lower than it really needs to be. Obviously it's better to be on the conservative side than risk a catastrophic failure but in most parts of the country the ambient temperature is going to go above this and it would render the device unusable outdoors. I can't imagine these batteries being terribly useful in any device PV or otherwise if they couldn't take more heat than that. That's just my best rational guesstimation though.
When I hear prolonged exposure I think like sitting on a dash in the summer being exposed to that temp. not operating temp. However, the ambient outdoor temp arguement does make a lot of sense. I know there are some battery experts in here. I just set it at 35 C because it was running at 32 C and when I did the upgrade to 1.1 it set the alarm to 0 so it was on all the time.