I wanted to get this into the answers section. @rodo found the answer right on the Xilinx/AMD website. This was a great find that eliminates any speculation.
In article 19146:
https://adaptivesupport.amd.com/s/article/19146?language=en_US
The topic of the article is how to make the part 5v tolerant. In addition to describing how, they state the rationale behind it. In their process, they come up with a 263 ohm resistor. I suspect that the Digilent engineer(s) were aware of this article and then used a 270 ohm resistor.