They can get to a "downed" state, but they cannot die.
Depending on how early they realized it was limited to companions on the ship, with a specific state (the source is not clear on how early they determined that to be) they could log events that damage a player when in that specific state.
That would have gotten them to fall damage, at which point they might be able to make the leap to ladders. If not, they could then focus their logging around changes in elevation when in that state.
But that's backward reasoning from the already known end state, so it's not fair to apply in hindsight. It's also not clear how much log data they can ingest, whether end-user telemetry was feasible, etc...
Their non-logging-based investigation got them to the point that they were aware the problem occurred when a companion fell from a great height while the player was on their ship:
> The only place in the game when a companion is present but not in the active party is when the player is on their ship
> Eventually we figured out that "undamageable" does not mean "invulnerable" -- they can't take damage from attacks but can still get hurt from other things
> One of those things: falling a great distance
> The problem with that is that there are no spots in the player's ship that are high enough to result in a lethal fall
> So now we had to figure out how companions were mysteriously ending up way above the level
> I looked into tons of theories
This does look like logging would have caught it. They knew exactly what they were looking for: companions reaching strange heights while the player is on their ship.
The debugging is to reconcile "Observation: quests are failing because companions die" which contradicts "My understanding of the game rules: companions can't die". One of the ways the companions don't die is that the companions are revived after combat, and it's only non-companions that are 'really dead'. The other way is "could die from fall damage on the ship, but there's nothing that high".
Turns out the flaw in the logic was that the NPCs can reach great height on the ship. But the flaw in the logic just as easily could have been in the combat. (Or some other way that the tweeter didn't mention).
In any case, it's really fun to read the "aha! got it" moment.
