Learn Finnish: On totta, että tämä argumentti on huono. - It is true that this argument is bad.

Good catch! While most truth-value adjectives taking an että clause do use the nominative (like on selvä, että or on varma, että), the word tosi (true) is an exception. The phrase on totta, että uses the partitive form, but it functions exactly like those nominative truth-value adjectives in this sentence structure.

Why is huono in the nominative case here? Doesn't the lesson say adjectives describing clauses take the partitive?

The special rule from the lesson only applies to the adjective describing the entire että clause (which is totta here). Inside the sub-clause itself, standard grammar rules apply. Tämä argumentti is a specific, countable noun acting as the subject, so the adjective describing it (huono) must be in the nominative case.

If we flip the sentence to say 'It is bad that this argument is true,' how would the cases change?

You would say: On huonoa, että tämä argumentti on tosi. Because 'bad' is an evaluative adjective describing a clause, it takes the partitive (huonoa). Meanwhile, 'true' now describes the normal, countable noun 'argument', so it takes the standard nominative form (tosi, rather than the fixed expression totta).

Is the comma before että strictly required?