I do not think the comparison stands between Retool, or this tool and Foundry. There is indeed a sub-product in Foundry called 'Slate' which is an UI builder but it's a small part of Foundry. Foundry is mostly about data pipelines, to do spark transforms on large ETL, and then having lots of product on top of it to make it easy to make Spark work in an enterprise environment such as a UI builder (the slate mentioned above), a graph viewer of the ETL (monocle), a report builder, RBAC, a timeseries processor, and so on.
I agree that the Retool comparison is only a small part of what Foundry offers, and that's my point, I don't know of any open source alternative that comes close to Foundry. They overlap in the sense that they are both tools that can be used as the back-office / operating system of a business, to varying degrees.
Foundry relies on Spark to do the actual ETL so they can focus on doing the products on top, the most interesting one is the integration with data lineage imho. But in practice, many business did not actually need big data since most of their ETL could run on top of a few non sharded sql queries on postgres. On the other hand, if you care about ETL there are a few amazing competing tools, dbt, airbyte, snowflake, the databricks platform and so on. And last, not being open-source is in my opinion a big risk for a large enterprises to bet and implement all their internal processes and golden tables on (or "ontologies" as palantir love saying). Even though Palantir would love to be product-led growth, their moat is strongly in their forward deployed engineer, half-consultant, half-software engineers that can push Foundry in big old-school companies and governments.
There is space for a new wave of less bloated, open-source tools and I for one am pretty excited about the new players in the field.
I agree, the website is terrible at explaining what it is, I only got it by seeing it fully implemented in a business. It's too broad to describe here but this might help: https://www.youtube.com/watch?v=uF-GSj-Exms
The comparisons listed (other than being OSS) are mostly superficial and in some cases already available in Retool, perhaps released since that was written.
A better benchmark to aim for would be https://www.palantir.com/platforms/foundry/ which is way more powerful.