I'm here to say that Suna and Osamu were in the same hs class and that Osamu told Suna first about not going pro, so I do think their interactions were deeper that what was just shown in the match. The "Ah, that" indicates that Suna already knew about it.
And considering how hard it must've been for Osamu to tell it to Atsumu, and that he told Suna first, there must've been a deeper level of trust and friendship.
Ginjima and Atsumu were classmates, too, class 2-2.