From my experience (and conversations with others who have landed TCs) the written assessment is not something that can on its own entirely eliminate you from contention.
It’s marked on several factors, in an attempt to gauge comparable skills of a potential trainee.
Things like: Can the candidate handle the time pressure? Can they complete a relatively complete and well structured email/memo. Are there glaring spelling and grammar issues? Can they follow instructions given in the assessment? Did they hand it in on time and in the correct format? How many legal/commercial issues did they attempt to extract?
I’ve done a few of these now and it has always stressed me out too, thinking back on how I did. What helps me is to just remind myself I did my best given the circumstances, and that assessors know it won’t and isn’t supposed to be perfect.