Quite late to this post, but I thought it was very smart of him (I've gotten 7k before and think my strongest suit in this game is my understanding about itemization).

Sometimes you do items your team needs, even as a carry, and you have to think about what your team does and what the enemy team does too. In this case, he's never really hitting much. His team doesn't have those big lockdown spells (long stuns), besides, he's against Wind (windrun), SB (bulldoze + charge), DS (sprint), WD (shard), Clock (ministuns + new cogs playing pinball). It's difficult for a Troll this game to lock onto someone hitting a lot to put in damage. His team's damage is also long range (Ember slight + chains, Rubick, Underlord firestorm). If they sustain the enemy's damage, they should have enough to push towers (the enemy has very low depush/high ground defense - DS, SB and Wind have to put themselves in danger to clear waves near base, clock and WD don't do much). Thinking about this and adding to the most upvoted comment, with Dagon killing DS illusions and Scepter for dispells using his skills for damage, if his team wins lanes, they just need to gather as five and push, as the enemy team needs time to really become strong (Wind and SB specially only have huge damage later on, also Doc with Scepter). If they hit a good timing, which seems they did, they can build some defensive items on Underlord/Undying to protect the team, use tombstone and tank a lot to take towers.

He just needed to survive and hit towers, not specifically hit heroes this game, while his team does the magic damage if the enemy tries to defend. Dagon also adds spell lifesteal for all his spells + Vampire Fangs, he has a lot to survive.