It's not full BC in anyway. Here is the breakdown from this analysis. 27% 360 titles and 3.92% OG Xbox. Far from full, especially on OG.
Edit: Maybe you just mean full BC to what is BC already from 360/OG on XBO/X. If so, then you're right!
Resetera BC Analysis
Total Number of BC Titles
575 Xbox 360 Titles
2103 Xbox 360 Titles Total (27.34%)
1900 Xbox 360 Titles Total Excluding Kinect and Peripheral Based Games (30.26%)
41 Original Xbox Titles
1047 Original Xbox Titles Total (3.92%)