Learn Spanish (Latin America): Seguramente el anfitrión estaría enojado.

In Spanish, the conditional tense isn't just used to say what someone 'would' do. It is also commonly used to make a guess or express probability about a past event. By using estaría, the speaker is conveying that the host 'was probably' or 'must have been' angry, rather than stating it as a known fact.

Since seguramente already means 'surely,' isn't using the conditional estaría a bit redundant?

It might feel that way to English speakers, but it is very natural in Spanish! You will often see adverbs of probability like seguramente paired with the conditional of probability to emphasize that the statement is a strong guess about the past.

Does seguramente mean 'securely' or 'safely'?

No, it is a bit of a false friend here. While the adjective seguro can mean 'safe' or 'secure', the adverb seguramente almost always translates to 'surely', 'probably', or 'most likely' when used in conversation.

Why do we use a form of estar (estaría) instead of ser (sería) with enojado?

Being angry is a temporary emotional state, which requires the verb estar. If you used sería, it would change the meaning to a guess about his permanent personality, implying he must have been an angry person by nature.