That doesn't follow at all from what I said. It's disingenuous to apply an argument made in 2016 to 1954. Things change, obviously. I'd also note that the original Yonge line did take reducing capital costs quite seriously (eg. large amounts of above ground track).
I also don't agree that the ECLRT is representative of 'light metro' in this context. The ECLRT platforms are very long, more than 2x the length of the Canada Line for instance. As rbt pointed out, station volume is a major contributor to cost, so this isn't a trivial point.
The latest estimate for the Arbutus extension in Vancouver is 1,900$m for 6km versus 4,400$m for the 7.5km DRL. Obviously tons of caveats are in order with that comparison. It's not definitive. But it is the case that Vancouver has consistently been able to build cheaper underground transit thanks to smaller, lower capacity systems.