The person who wrote that letter to you didn't know what they were talking about.
FTM's Relationship Calculator attempts to find ALL relationships, by blood or marriage. I just looked up an ancestor on my dad's side to an ancestor on my mom's side - who are not related by blood (at least in my database), and got:
xx is second grandfather of husband of 8th great granddaughter of zz.
There are five other non-blood relationships between those two people offered up by the calculator.
That said, the problem is that the calculator is imperfect. There have been many posts to this board where a non-blood relationship is calc'd first and appears in the relationship box at the family page - and other blood relationships are ignored in the relationship calculator. I have had one person in my file that was shown as descended in eight ways from a singel ancestor in the Outline Descendant Report, but only seven are shown in the relationship calculator.
They really need to hire better folks in their support area or train them better on their own software.
Now that ancestry/ftm will be privately owned, it is up in the air what direction the new owners will take with FTM. Will they will pour all of their resources into ancestry.com?