"He is not happy, in other words he is sad."
Translation:Li ne estas feliĉa, alivorte li estas malĝoja.
"Si" is a reflexive pronoun. Meaning it's usually used when someone or something is doing something to itself.
"Li ne estas feliĉa, alivorte si estas malĝoja." could be translated as "He isn't happy, in other words himself is unhappy.", which doesn't really make sense.
A sentence like "He makes himself unhappy." could be "Li malfeliĉigas sin."
In fact as far as I can tell "si" is never used as a subject. It's used as an object i.e. "sin" and after prepositions. You can see this for yourself by going to tekstaro.com and searching for "\bsi\b" (without the quotes), which will bring search for "si" on its own (rather than within other words).
"Si" refers to the subject of the verb. As such, "si" can never be the subject. The sentence under discussion here is made of two complete clauses each with its own verb.
You don't, it just has not been added as a possible solution yet, report it and they will fix it.