I found this tweet [1] by the OP to start answering my own question.
In short, while both InstantDB and Zero use PostgreSQL as a backend (and add a subscription mechanism in front), the former uses it as a triple store (more similar in practice to something like Triplit), meaning the data is not available in a format comprehensible to other clients, so it is merely an implementation detail. Zero however allows the database to be accessed simultaneously in other manners, promoting compatibility and reducing lock-in.
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I found this tweet [1] by the OP to start answering my own question.
In short, while both InstantDB and Zero use PostgreSQL as a backend (and add a subscription mechanism in front), the former uses it as a triple store (more similar in practice to something like Triplit), meaning the data is not available in a format comprehensible to other clients, so it is merely an implementation detail. Zero however allows the database to be accessed simultaneously in other manners, promoting compatibility and reducing lock-in.
[1] https://x.com/aboodman/status/1843049082237170036