DoIP Dragon Barrow Area D5 – how is this not a likely TPK?

I’m running Dragon of Icespire Peak, and my players have encountered the Dragon Barrow.

This question relates to the mechanics of a specific part of the quest, so I’ve liberally used spoiler blocks to avoid spoiling this for any players of the campaign who might find this question.

If you’re playing DoIP in any group (not just my players), don’t read this question or the answers.

Section D5 is described as (emphasis mine):

Given that

Because

Am I missing something, or is this likely to lead to a TPK, with no particular warning for the players?

Obviously as DM I can overrule and say that

but nothing in the module as written suggests this. I suspect I’m missing something?