Mo Tiange didn't explain it to him. In any case, she'd long rejected his Dual Cultivation proposal and she also wouldn't have too many interactions with Tang Shen in the future, so she didn't need to explain it that much.
Tang Shen also didn't bring up the matter anymore. Instead, he smiled and said, "That being the case, I should also call you senior martial sister. Senior Martial Sister Mo's so young but already has such a high cultivation level—calling you senior martial sister's what I should do."
In the cultivation world, seniority was never determined based on age. Only people who were closely related to each other like martial siblings under the same master would use age to determine their relative seniority and maintain that manner of address despite any cultivation progress out of respect.