Only by the usual definition of "heroic = incredibly stupid". Why choose to die when she could just have stepped out of that mysteriously blastproof door (mysterious because somehow no other door or wall or floor or ceiling in that room was blastproof, including the one that supposedly worked as it should) and then pulled the fateful lever, with a piece of string. Or, you know, just performed a hero roll below that veeery slowly descending door?
Since she was the heir apparent to command S31 with the demise of all the other onscreen black badges, though, perhaps she merely committed suicide in order to dodge the duty.
But what's this about S31 being an organization that "does not officially exist"? There's none of that in DSC. Everybody knows that S31 exists (and wears black badges). Everybody knows what S31 does. It's just that nobody knows what S31 does, specifically, so it is just a tad mysterious and shady and all. And perhaps at some point of time, S31 is supposed to cease to exist (say, right after the UFP High Council hearing on the Control disaster), so it continuing to exist in the late 24th century is somewhat unexpected.
I just wonder what other branches of Starfleet have their own easily identifiable badge types. I mean, TOS was rife with distinct badges. At the very least, DSC could have shiny, black, red and striped arrowheads...
Timo Saloniemi