That's because you're right and he's wrong. He's a well known bitcoin core apologist, and is just talking his political position. The original intended system design at scale had no need for non mining nodes.
This revisionism is a long running and well known war, here is the core political bloc in question actually attempting to justify editing the white paper to push their perspective over reality.
For those of us that only follow the latest on bitcoin at a high level, I think it is difficult to determine what are the points of disagreement. When I first read about bitcoin on HN in 2010, and read the whitepaper (and went to their IRC channel), I certainly don't remember thinking that non-mining nodes were that important. However, now I'm not sure, and many people making arguments on all sides seem to have good arguments at various times. I am hoping more people with good knowledge of the issues weigh-in here, rather than the over-politicized subreddits.
The real truth is that both everybody and nobody is in control.
Every participant in the system has a part to play and has a choice, miners choose which chain to mine and in doing so secure that chain from attack and earn a return, people that run nodes pick the nodes to run based on which chain they believe has the most value, or in the case of miners in order to mine the chain they believe has the most value, spv wallet users transact on the chain they value and pay fees to support the upkeep of it, the simple act of conducting trade with a cryptocurrency gives the entire apparatus basic value, so the people that don't give a damn about any given blockchain and just want to use it to move some completely unrelated asset from a to b also still give the system value, and of course the holders and traders of the actual blockchain assets give the system value and play a part in resolving contentious forks, by evaluating what they see as the market value of a given blockchain asset, forked or otherwise, and profiting or losing based on that insight.
Short of outright theft of a private key, nobody may compel even the smallest user of a blockchain to perform an action that they do not freely wish to undertake, not all the devs writing node software, shills pushing political agendas, or even miners mining blocks in the chain can change that fundamental aspect of the system that keeps it actually properly decentralised. The only way around this is to drive the vast majority of transactions off chain and force most end users to operate through third party intermediaries that manage their actual potential transactions in the system.
Like exactly what core are doing with the lightning network, for example.
To be quite honest, it doesn't matter if folks think that non-mining nodes are unimportant - they can't force folks who /are/ running full validating non-mining nodes into accepting arbitrary rule changes to the protocol. That's what's important.
This revisionism is a long running and well known war, here is the core political bloc in question actually attempting to justify editing the white paper to push their perspective over reality.
Cancerous stuff. https://github.com/bitcoin-dot-org/bitcoin.org/issues/1904