An even less significant rounding error; on the order of ~0.002% relative to AWS carbon footprint at best using FY23 forward net sales as a rough proxy.
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.