We're using the highest tier Postgres instance at my work for one of our legacy Heroku apps and it costs thousands over what we'd pay for the equivalent on AWS directly.

Sure, but those are not related to Aurora or this post.

Um, what? It's literally what we're talking about haha

This post is talking about plans that are at most $20/month. I don't believe the other Heroku plans are on Aurora.

I know Salesforce has a huge AWS presence. That said, is it possible they are doing multitenancy? I don't know myself.

According to https://elements.heroku.com/addons/heroku-postgresql the instances they're using for this tier have zero bytes of RAM, so presumably that's where they're getting most of their cost savings from.

I'm assuming this means that they are not providing any sort of guarantee on the amount of RAM available and packing these instances as tightly as they can.

I like to think they just aren't installing any RAM in the servers and running all the databases out of L3 cache

“Amazon Aurora Serverless is an on-demand, autoscaling configuration for Amazon Aurora. It automatically starts up, shuts down, and scales capacity up or down based on your application's needs.”