It's a bit weird since it would make sense as an idiomatic expression too, purely because the word "koti" (home) is used instead of e.g. "asunto" (apartment). "Koti" is more abstract than for instance "talo" or "asunto", and refers to the feelings you have about a place. But it must be English affecting my interpretation, since I could easily use "lämmin koti" in the idiomatic sense. A place can have a "lämmin tunnelma" (warm/welcoming atmosphere) in Finnish as well.
Isn't Teillä here the second person possessive plural? In which case, it would be right to sometimes insert an extra "all", just as I might with German ihr...right? In fact if I mouse over teillä in the question itself, "you all" is given as a possible meaning. What am I missing?
The 2nd person possessive plural is "teidän", though it would be more accurately referred to as the 2nd person genitive plural, since genitive has more functions than just indicating possession. "Teillä" is 2nd person adessive plural. Inserting an extra "all" is needless because "you" is already plural. The probable reason as to why "you all" is given as a possible meaning is the fact that merely "you" could be misleading due to its ambiguity.
In other language courses here on Duolingo (e.g. German), the translation "you all" or "you guys" is accepted in such a situation. From a pedagogical perspective, it should be actually preferred as an aid for native English speakers to differentiate the 2. person singular vs. plural through association with something else than the plain ambiguous "you". Therefore, I believe "you all" and "you guys" should be added here.