With an exception to OVH, none of the cheap providers the article has listed have any kind of backbone network. They all rely fully on transit providers. Turns out backbone networks are expensive to operate!
OVH is the sole example of a provider that has a backbone network, and admittedly, it's pretty good. However, nowhere near expansive as the big three, and it falls flat in Asia (which is the hardest to route traffic in). Also OVH has to build datacenters so cheaply that one of them burnt to the ground in recent years...
(Cloudflare has a backbone too, but you have to pay a lot extra to use it. Linode uses the Akamai backbone now but that's a very recent acquisition and it's expected that Akamai will eventually raise costs significantly)
Yes, bandwidth is way too expensive on cloud providers. AWS Lightsail is proof of that. However, I see no reason to believe that this is purely for vendor locking, and nobody has been able to give any evidence of causation between the two beyond "well it's so expensive!!!"