Not untrue, but also not true. It isn’t a Lambda problem, It’s the problem of running the same old, same old code on lambda.
Running clusters of VM/container/instances to “scale up” to transient demand but leaving largely idle (a very, very common outcome) then an event oriented rewrite is going to save a big hunk of money.
But yeah, if you keep every instance busy all the time, it is definitely cheaper than lambda busy all the time.