GPX files produced from FIT files in BaseCamp and Connect look significantly different from GPX produced in the unit at the time of recording. However, my initial analysis suggests that GPX files exported from FIT files by the unit itself may be identical in format to those recorded directly.
Here is a GPX exported by the 66sr from a FIT file recorded weeks earlier. I went to Recorded Activities, opened the activity, and then used the "Save as Track" command. (The 66sr was set to only record FIT, so I know that the resulting GPX was converted from FIT.) The only difference that I see, relative to a GPX recorded directly, is that the <metadata><time> field shows the time of conversion, not the time of recording. The decimal precision and overall structure look the same to me. I'm not saying that this is equal to a GPX produced at the time of recording, but it looks the same and hasn't been tampered with by any software outside the GPS receiver. (Wrapping is different due to a different text editor; the original file has no line breaks.)