Star Trek Discovery: Every Easter Egg & Hidden Reference From 'People Of Earth'

Michael is surprised to learn that the Trill are a joined species after it's revealed that Adira is a human joined with a Trill symbiont formerly known as Admiral Senna Tal.

This is actually a really nice nod to a confusing bit of Star Trek continuity:

While the Trill have appeared in previous episodes of Star Trek: Discovery ("Will You Take My Hand?") and apparently interacted with Starfleet and the Federation since at least the mid-23rd century, their status as a symbiotic race was only revealed in the late-24th century in Star Trek: The Next Generation's "The Host".

It's not clear how this information remained unknown for so long, especially considering Benjamin Sisko's close relationship with Curzon Dax and the several Trill who joined Starfleet, but it's always possible some people knew and kept the Trill's secret. Either way, this knowledge is present in the Sphere data the Discovery downloaded in "An Obol for Charon" and will likely come in handy as the season progresses and we learn more about Adira and Admiral Tal.