None of their mounts are spec'ed to over 60 or 70", I think they are just out of date. The mounting dimensions are the same on the 80" as the smaller models. My TV (LC-80LE844U) weighs 113lbs and the mount goes to 165lbs so it should be fine. You can bet I will take every precaution when I do the install - if it doesn't look right I will back off. I used their mount on my 60" sharp upstairs over a fireplace and it worked great.
The low profile mount I used on the 70 is rated for 200lbs. The 70in weighs like 68lbs so it is well within spec. The size guides Monoprice uses is based on the vesa mounting pattern. As long as the mounting bolts line up it is fine.