LingQ itself doesn’t assign CEFR levels. 13,882 English words known would put you in the Intermediate 2 level on LingQ, but that is not B2. There are some posts on the forum (one linked below) from users who have experimented and say those word counts do correlate to the CEFR. However, as you point out, this would depend on how one assigns “known words”.
Tests like Hsingh pointed out can give you an idea for vocab.