Since there are really many companies and people using AWS, the effect would be noticeable if everyone managed such a decrease. Calling it "an even less significant rounding error" borders on bad faith.
Your condition is predicated on "everyone" at Pinterest revenue scale justifying the business case of throwing away their existing tech stack and adopting Elixir with an equivalent architecture---never mind that the article is coming from a company that sells Elixir consulting services---while major CSPs voluntarily bend over to notionally substantial revenue decline...and I'm bordering on bad faith??
At least I bothered to put a supporting numerical estimate on the hypothetical; you're just handwaving greenwashed bullshit with improbable, unsupported outcomes.