That waymarkedtrails route has both the original start via Loch Treig and Loch Ossian and the current start which goes via Glen Lyon, Loch Rannoch and Ben Alder.
I used OSM maps this year and didn’t have a problem with the actual route but I know that the mapping is patchy/inconsistent in our neck of the woods so there’s every chance it is in other areas. I’ve not investigated the accuracy of tracks to the side of the route, i.e. escape routes.
It should be noted that even OS maps have their shortcomings – the infamous black dashed line can be anything from a landrover track to non-existant on the ground, sometimes on the same track!
There’s plenty of on-line resources, Google maps and streetview, Geograph.org.uk, among others, that you can check to get an idea of what the terrain is like. That’s before you get to individual blogs, websites and accounts. No real reason to head into something like this blind.