I mean, I agree, saying "teleport" when that's not a defined term is problematic- if they mean like the spell Teleport, they should say so. But it's not like this is the first or last time that WotC kind of leaves us to infer things. Because then we have to ask "how long does it take to Dimension Door?"
Well obviously, when you cast Dimension Door, it's still your turn afterwards- you can still move or take a bonus action. You could ready an action for someone using Dimension Door to "teleport" away. So it really comes down to the crux of the argument.
If I take a reaction after someone uses Dimension Door, are they now at their target location or temporarily removed from play? What's making me come around on this, despite the fact I still disagree that Thunder Step is balanced if it includes you in it's area of effect is that the rules don't say you are "nowhere".
They don't say you are "somewhere" either, but if you are meant to be "nowhere" for a brief span of time, the rules probably need to say so. Because it could come up.
But "removed from play" isn't a condition in 5e. There is no zone you can be exiled to. Even with things like Banishment, you are "somewhere"- and the only defined location you can be, is the target location.
So let's look at it this way.
Line 1: Cast Thunder Step, targeting an unoccupied space you can see within 30 ft.
Line 2: Teleport to the target space.
Line 3: 3d10 Thunder damage (Con save 1/2) dealt to each creature within 10' of the space you left.
There's not much wiggle room here, since we have no guidance for when you arrive at the destination. What we can infer is that there is no reason for travel to not be in the smallest available unit of time. We also have no guidance for how long it takes the blast to deal it's damage. If we infer this also takes the smallest available unit of time, then it occurs immediately after you teleport. Thus-
Line 2: "Teleport" (leave play and arrive at destination in X zeptoseconds).
Line 3: Deal damage to creatures within 10' of the space you left in X zeptoseconds).
If we assume that there is an x zeptosecond gap between Line 2 and Line 3, you can find yourself in the area of effect of the spell. Only if we assume there is not an x zeptosecond gap between the first and second effect can the target arrive safely.
There is no guidance to suggest such a gap does or does not exist, but again, you can't definitively prove it either way. This is why, against my feelings, I am fairly certain that any official guidance given will say that the effects of the spell occur sequentially with no time elapsed because 5e is not designed to be this precise.
"Time" is a hazily defined, nebulous concept. We can roughly estimate how long a round is, because there are effects with durations that need to be tracked. But how long individual events within that round take is left completely undefined, and left up to the DM to rule on if it becomes necessary to do so.
Thus while we can demonstrate there could and even should be a gap in time, no such gap is stated. Anywhere. So nobody can be right or wrong here, but again, since 5e is not built to require this level of precision, all we can do is state our hypothesis, and if it is rejected, accept that we cannot convince someone to believe in something they cannot observe in action.
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