Not sure if I'm correct, but I've reported this sentence as "the hints are wrong/are missing" because it would be better if they said "it is", rather than "there is, is, am" under "tha". Hope that's ok. Let me know if I shouldn't do that as I wasn't sure if it's right to do that.
Tha on its own doesn't mean 'it is', you would need an e or an i for the 'it' part. If you add a hint to an individual word, it adds it to every sentence that word appears in. That's why is and am appear under tha. 'It is ... ' does appear in the hints, but it has been added to the 'tha ... ann' construction. Does that make sense?
It is not necessarily a limitation to gloss tha as am/is/are anyway. If it were given as is in this sentence, you might be given the false impression that that is what it always means. You need to learn that it means am/is/are and that you have to choose which from the context.
In some languages, such as Latin and even Old Irish there is a word that means 'he/she/it is' so you do not need a pronoun and you need a different word for am or are. But as Old Irish morphed into Gaelic, the system changed, so tha can be used for any person and you do need a separate pronoun. D