The approach they take is one described by Stephen Robinson in Are Mormons Christians? Some categorize Mormons as non-christian by what he calls the doctrinal-exclusion argument, somethig like this.
2) Belief x is not a belief of historical Christianity.
3) Therefore Mormons are not Christians.
Peterson and Ricks make the following logical argument.
1) Mormons believe x.
2) Prominent theologian Z in century w also believed and taught x, and he/they are always considered to be Christian
3) Therefore, Mormons cannot be excluded from being Christian for holding belief x.
It’s a fairly simple framework, and an effective counter-argument. Mostly.
I’m not a fan of how the menus work on the FARMS page. Linking directly to the book doesn’t allow you to access other chapters without clicking on multiple links, since whatever section you click on replaces the full menu of all books. Clunky it may be, it’s good to see it online.