I've been learning Dutch on duo for over a year and zij/she vs. zij/they always trips me up, despite conjugation which should make it simple.
I find "besloten dat" a very weird combination. I mean a sentence like "Dit doelpunt zal de wedstrijd besloten hebben." sounds OK to me, but with a person being the subject does not. If someone, possibly a native speaker, could help me out with whether my gut feeling is right or wrong, please do.
It's perfectly normal. Dutch and English are actually very similar in this sense, e.g. een beslissende actie (a decisive action), dat was besloten in de vergadering (that was decided in the meeting), ze besloot te gaan (she decided to go).
Next to that Dutch also has besloten in the meaning of a closed/private thing, e.g. een besloten vergadering (a private meeting). So this is a normal sentence: dat was besloten in de besloten vergadering (that was decided in the private meeting).
In cases where they mean to make a decision (like in the exercise sentence) there is no difference, however beslissen can also mean to be decisive/to decide, e.g. Zijn doelpunt besliste de wedstrijd. (His goal decided the match), so the dicision was made, but the match probably didn't end yet. Besluiten can also mean to conclude, e.g. Zijn doelpunt besloot de wedstrijd (His goal ended/concluded the match), so his goal was the last action of the match.