Well, he did finish fourth in the TdF this year, just as a certain Slovenian did in 2018. But yeah, he would have to be considered a pretty long shot, although Bora's strength this year has caught me by surprise. On the right course he could be at ... Kruisjwijk's level...which I know isn't a ringing endorsement...
As somebody else said, Buchmann was already better this tour than SK (which was the very best version of Kruisjwijk), he would have finished in front of him if not for the TTT.
He has improved every year since 2014, his increasing World Tour points tell all the story pretty much. Question is has he peaked? If he can take another of his regular yearly steps forward he will be among the absolute best GT contenders, that is how close he is already.