No, it shouldn't - um thráthnóna is a widely used phrase that everyone will interpret as "in the afternoon/evening"
You use thart ar for the around/about sense of approximate times, though tráthnóna is already so non-specific that "around evening" doesn't really make a whole lot of sense, in English or in Irish.
I would like more information on this. as an English speaker this is seems more likely to be evening because that is the meal in the latter part of the day. While lunch is midday-ish. But evening can mean the early part of night for me which isn't covered by trathnona. The period between noon and night seems very long, particularly when in summer night may come at a very late time.
The midday meal can also be "dinner" - the prevalence of "lunch" at midday, and "dinner" latter in the day wasn't always the case, and at least in Ireland, the evening meal is still "tea" in many families (teatime doesn't just mean a cup of tea and a biscuit), though dinner is also widely used.
Even the Wikipedia article on "Dinner" says:
Dinner usually refers to the most significant and important meal of the day, which can be the noon or the evening meal.
The chart doesn't need to be updated, and it would be misleading to suggest that people can use um in most cases where they want to say "in the". Prepositions often have idiomatic uses in different languages that don't translate directly - even in English you would change the preposition to "at" if you changed from "evening" to "night". Why? It's just a habit - the semantic function of the preposition is the same in both cases.
Learn that most English speakers say "in the afternoon" or "in the evening" where an Irish speaker would say um tráthnóna, and that an English speaker would usually say "at night" where an Irish speaker would say um oíche, and that an English speaker would usually say "on Christmas Eve" where an Irish speaker would say um Oíche Nollag rather than pretending that um means 3 different things.