At first I thought Tadokoro would be sad and angry at Nikaidou because, when asked about their relationship, Nikaidou denied that they are girlfriends and said they are just friends.
Then I realized that Tadokoro and Nikaidou are probably on the same wavelength when it comes to coming out... Tadokoro is prolly very happy that Nikaidou kept their relationship out a secret.
I'm not sure either of them saw it as denial or "keeping their relationship a secret". She didn't say "We're just friends, nothing weird nope nope nope", she said "Our relationship is VERY FRIENDLY". Just how friendly? Friendly enough that she was holding Tadokoro-san possessively as she said it. I think in her mind she was saying "She's mine" in a less-embarrassing, subtler Japanese sort of way, kind of like one might indicate "I love you" by saying "The moon is beautiful tonight".
And Tadokoro-san probably more or less got it and was happy. It's just that eyepatch girl is not mega-subtle and didn't catch the signal.