Sure it could just establish that the Byers will visit Hawkins from time to time in the time between seasons, but that could’ve been said without specifically mentioning Christmas.
While it’s not clear evidence a Christmas special is in the works, it certainly is interesting to say the least.
That’s an oddly specific line to throw in there, and one that Stranger Things probably could’ve left out if writers weren’t trying to imply a Christmas special is on the way.