The Forerunners would not have wanted to honor Mendicant in any way, so why would they build a tomb or write an epitaph for him? Instead, I think that Epitaph could be a monument for Offensive Bias who they created to combat Mendicant Bias and Gravemind. They constructed the building that Epitaph is at the top of to honor/store Offensive Bias which would have been seen as a savior of sorts. This may have already been thought of, but all I see is "Oh, Epitaph is Mendicant Bias's tomb! Let's go find the tomb everyone!" so I thought this would be fresh.