cutting the frame down

I have another question for you guys! I asked over a week ago about finding a unicycle short enough for me to ride (What size for a little person/dwarf?) and you guys were super helpful. I found a lightly used 16" Corker CX on Craigslist for a great price.

I had the seat post cut, but I still need the frame shorter. I know from reading a post here this can be DIY, but I lack the tools and know-how to do it. I have asked two different bike shops here and they said they cannot do it. They both said I should take it to a welder. I didn’t totally grasp why, but I think it has to do with the seat post clamp (which is integrated). I guess I don’t understand why the frame can’t be cut and a new clamp put on?

What do you guys think?