Either the burning in has been exceptionally well executed given the difficulty involved or it is simply the exposure which has given the silhouette effect. To get the dark but detailed sky I think it may have been a combination of the filter and exposure for the sky in a contre jour situation.
This is getting close to a critique which hasn't been requested but I'd be interested in whether burning in was involved.
I'd have sacrificed detail in the tree for the sake of the rest and maybe printed at slightly more contrast than normal which is what I think was done.
I split-grade print everything - so i don't know what grade it actually is, but it's fairly hard. The sky was heavily burnt at grade 00, the foreground was (i think) about 6 seconds at 00 and 12 seconds at 5.
Looking at the image now, i wish i had dodged slightly during the hard exposure on the right to even it out a bit.