My guess is Umi took flirting tips from Nozomi and Eli believed that it is Nozomi held her hand there to tease her friend but.... it now got awkward for Umi lol.
Admit it people with how ~affectionate NozoEli are, Nico has it the worst with thirdwheeling. And the fact that her Tsun partner is well... too Tsun doesn't help much.