It's a relief landing ground so any aircraft could use it. Equally likely of course is the idea that the C17 was actually picking up troops there.
I knew they used it for training with light aircraft like Tucanos, but I was surprised to see a C-17.
I live not too far from what was once RAF Ballykelly in Northern Ireland. After the RAF left it was still used regularly for assorted military activity. Helicopters, spotter places, training flights etc. You would occasionally see Hercules flying in and out. One day a Ryanair scheduled flight from Liverpool landed there by mistake. The pilot was evidently making a visual approach to the City of Derry civil airport a few miles away but mistook Ballykelly for the correct airport. I dare say there was a bit of paperwork following that slip up. See:
So just because the RAF have mostly left, who knows what aircraft you may see at Leuchars.