I guess we're hearing different things. Sunn is a drone band in the style of Earth's Earth2 album (the start of doom metal influenced drone music) who now has more a black metal influence (see Black One). Boris started out doing Earth style drone with a Melvins influence (their name taken from a Melvins song off of Bullhead) and have gone on to include more of punk influence (see Pink) and 80s Japanese metal (see Smile). Isis is pretty much a band HEAVILY influence by Neurosis and Red Sparowes takes that sound and gives it more melody with more beauty (and less brutality) but neither is close to drone music.
My take on post-rock is that it was progressive rock for the independent scene that came from the post-punk bands (see Slint's Spiderland album as heavy influence on post-rock) who couldn't call it prog rock because of the connotations of it the name.
For me, no one has been able to top the sound that Godspeed You Black Emperor has created. A few bands come close. I haven't heard the names of Sparrows Swarm And Sing or Mountain Men Anonymous mentioned so I'll throw those out there.
YES! It's indie-prog, in the sense that is it pretty, epic, expansive, "progresive" (not so much technical, though). Now I realize why I like post-y stuff.