It's a documented part, the OP was just confusing two different parts/part revisions.

The paranoid would say "it's a hidden backdoor for running shady code". However, anyone with even a passing interest in embedded computing would recognize that it's designed to store and run official firmware; nothing shady or underhanded about it.

So in short, in this particular instance it's much ado about nothing.

The hardware doesn't care what it was designed for; it does what it's told. It is extremely plausible that capabilities which exist for testing purposes can also be exploited for running malware.

Correct, but the implication in the tweet was that it was secretly embedded by the manufacturer for nefarious purposes. The Twitter OP jumped to paranoid conclusions in part because of their obvious lack of understanding of even simple embedded hardware concepts.

You can kill someone with a hammer, but that doesn't mean the hammer was designed for killing.

Two different parts build using same silicon. Standard industry practice. ESP8266 started its life as a slave SDIO network card with undocumented master mode.