You are correct it could carry ~ 971k BTU that 150' distance, but you have to remember that fittings (90s etc) add a lot of "resistance". I think there is a note on the page that you add 50% of the actual distance to account for the fittings.
In reality, what we are discussing is slightly over simplified and on the conservative side. There are more detailed methods that can be used to determine the pipe size. I can not recall what website I looked at previously with more detailed methods when I was considering having my house re-done.
You will have to dig a trench regardless of the pipe size, so using a slightly larger pipe may not be much of a $ increase since the amount of labor is the same.
Again, this is assuming that you are going to get the large pieces of equipment that I mentioned ... many grills, fire pits, and heaters are smaller than I used for the estimate.
EDIT: one other note. That table is for steel pipe ... often now a PE (?) plastic pipe is used which is quicker and easier to install, but requires special tools. There are fewer 90s since the pipe can bend into the risers, so the carrying capacity may be different.