the trust comes from the number of nodes and how decentralized they are though, running one node yourself won't make your transactions more secure I think.
You are mistaken. That is exactly Adam Back's point. Read what he wrote again.
"it’s not just running a full-node, you have to actually use it for transactions"
What do you think he means by that?
He has also warned against renting nodes in data centers, which is exactly what you get when you pay people to do it.
"It’s the amount of economic interest that is relying on full-nodes and has direct trust and control of those full-nodes"
With this statement I interpret it that Adam is saying you need to tie-in the economic interest of the full node operators to ensure the security. Using your own full node is one way to achieve that, but operating a masternode for other users is another way because the operator doesn't get rewarded if they don't provide a secure service in the interests of the end users.
About not renting nodes in data centers, if Bitcoin increased to 20MB blocks, on current broadband etc, I don't see how 20x the throughput can work in the foreseeable future on consumer hardware so there is no choice but to scale-up the node's hardware and use datacentres.
It's how you keep a datacenter-based full node network decentralized is the big question, and Bitcoin full node network is already becoming centralized - again I think Dash is showing the solution because virtually all full nodes are on server hardware and decentralized across a lot of hosting companies around the world, because a user anywhere in the world can get paid to operate a decent node.
I edited the post above with a link where you can listen to the interview where he explicitly says that renting nodes in a datacenter and do do transactions is "quite useless". You can agree or disagree with him, but that's what he says.
As for 20MB blocks, he's against that, so his opinions are consistent at least.