They're right outside Maki's workplace and we already know Tsucchi works somewhere else. We've also seen from chapter 17 that Tsucchi doesn't hide her long hair under the coat and that she wears gothic boots. Also, you can clearly see the short hair of the mysterious back hugger in page 19 and 20. She's just a random they added to create additional drama.
The workplace thing is not dispositive--we've seen Tsucchi haul Maki around places, and she could plausibly have met up with Maki at her workplace in order to comfort her friend. But the hair/shoe thing is much more convincing, and if that is Tsucchi, the artist is intentionally muting/disguising her characteristics in order to set up the faux-cliffhanger.
So I'm now inclined to think that you're right. Since a quick flip-through of the rest of the series doesn't show anybody remotely suitable to be this back-hugging co-worker, that means the writer has conjured up another character out of thin air simply in order to:
A) create a bog-standard misunderstanding (Liz turns away, Maki spots her and chases after her to explain It's Not What It Looks Like, and then probably apologizes to Liz. Bonus points if Liz then slaps Maki.)
B) actually introduce an entirely new character who has the hots for Maki and Wants Her to Be Happy (with Her), creating a whole Maki Starts a New Life arc, until Maki realizes that Liz, yadda yadda yada.
Either way, we remain in Yuri Manga Hell.