That MIGHT be true, but it's not apparent from their statement. Just because it costs N to build a product on technology X and costs M to migrate an existing product to technology Y does not mean that it would have cost N + M to implement the product on technology Y in the first place.
I think the point made above is that it's enough for implementation(X) < implementation(Y) in order to sway some people to technology X. Rapid prototyping is an example of this, since the time-value of feedback is higher before the product is well defined.
You are correct of course, my point is that even if cost(implementing Y originally) < cost(implementing X and later migrating to Y) it may still be worth it, because the cost has been deferred. Time is often a lot more valuable in early stages of a startup.
