IMHO he is probably correct. If you want something like that go with the portable with dual venting., I have one it works great, You can use a window or make something for the garage door for the vents.. I just cut two 4" holes in the wall. One draws in the air the other blows out the hot through 4" hose. That way dust is far less an issue. The split has little filters and no matter how little dust you think you get it is way more than the split ductless are designed for. On the dual vent units you can actually just vent the hot out, but why draw in the air from the shop with all its dust when you do not have to.
Thanks I will do some research into that.