Q. I purchased a Threadleaf cypress which is outside plants in my 6 zone area but the vendor told me it was an indoor plant
"thread",... cypress usually refer to cultivars of Chamaecyparus pisifera with threadfeaf foliage. There are many cultivars and they are usually hardy here in the Northeast and are rated for zones 4 to 8. Hopefully, your plant is tagged and you can look up the hardy information. Again, you need to gradually introduce the plant to the outside this time of year and that may be why the vendor indicated that it is a houseplant.. If there is a tag and it says hardy, I would be inclined to believe the tag rather than the vendor.
Around the Holiday season there a number of thread type cypress plants sold as holiday plants. They are seldom tagged. These are usually European cultivars that are not hardy here.