It's pretty explicit in the books too! They're definitely called pillow friends at one point, which is Aes Sedai code for lesbian lovers. This is one of the changes I expected them to make for the show - keep them lovers and not have them end up with guys later on.
I always thought the term "pillow friends" left just enough ambiguity that it implied more than anything else. Of course it always made sense there would lesbian relationships in an entirely female organization filled with powerful, driven women. I don't think all the supposed "pillow friends" were lovers though. It think it was also used as a somewhat disparaging term for women in the Tower who kept close confidence with one another.
But also I mean it in the sense that there were no explicit sex scenes between them. Although I read recently that there was a point where they kissed in New Spring? Jordan though, as far as I remember, never really got graphic with any of the relationships in the series; he was more of a "kissing and then fade scene out" kind of writer.
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