My impression is that Kanoko's main issue here is that she really doesn't have a framework for close friendship outside of 'girlfriend' or 'lovers'. What she seems to want most, and is struggling to define in a context outside of a relationship, is to repay the kind of close support that Sumika gave her after Goeidou assaulted her.
I think part of why she keeps calling Sumika an 'idiot' is partly because Sumika keeps rushing to put herself into a self-sacrificing role, while cutting off any opportunity for Kanoko to explain herself. Notice on both page 3 and to a lesser extent on page 16, Kanoko gets pretty quickly cut off while Sumika redirects the conversation towards romance. To an extent, Sumika is even denying that Kanoko could like her ("I knew it would never be directed towards me") when Kanoko has given her the kind of smile she had been directing towards Hime on multiple occasions.
All this said, Sumika is well within her rights to try to limit her relationship with Kanoko because it's obviously painful to her to be close right now. She just hasn't communicated that well, and keeps talking about being there to support Kanoko in her relationship with Hime going forward.