"Bilking" implies deceit, and I don't appreciate the accusation. I've worked with forum/cms platforms, Linux and software development since the 1990s and built/supported hundreds of Xenforo communities, including client referrals from Xenforo staff.
Yes, changing a single font size in CSS can be trivial. However, a 1:1 typography match from Vanilla to Xenforo isn't "change a number". The non-trivial part is matching the Vanilla platform's many page types and each type's varying element level font sizes to Xenforo's style system properties in a 1:1 way. That means mapping dozens of individual element font sizes to the correspond style property setting in Xenforo (there are hundreds to go through). Then validating the results as each change is made.
To protect the client's budget, I took a pragmatic approach by adjusting global font sizes and font face to improve readability without the greater number of hours required for 1 to 1 parity everywhere.