Normally, there is a definite difference in meaning between "will have done" and "must have done".
"Will have done" is the future perfect tense, and it is usually used for discussing conditions in the future. The verb indicates an action that hasn't necessarily happened yet, but it appears from some future point in time to be an event of the past. Example:
By the time you read this note, I will have boarded the train for
"Must have done" is for a belief you have in the present that an event happened in the past. Example:
His toothbrush is gone, and he left this note. He must have gone back
to his wife in Kansas City.
However, the quote in your question does not seem to be a normal use of "will have been". From what I can tell of the context, I take the speaker's meaning to be synonymous with "... that must have been a very deliberate decision," or "... that would have been a very deliberate decision."
He is speculating in the present about a past matter that he did not witness.
So why did he use the future perfect tense?
Perhaps because the speaker is imagining in his head a time in the future when everything will be made clear, and at that time it will be known that the decision was very deliberate.
However, these modal verbs (could, can, would, shall, should, may, might etc.) are pretty tricky. People sometimes use them not for their literal meaning, but to express formality ("I should like to ..."), politeness ("could you perhaps ...?"), or equivocation ("May we suppose ...?"), and their uses sometimes differ among different varieties of English.
So it may be that in this gentleman's brand of English, he feels "that will have been" is more formal, or more appropriate to such a learned topic, than "that must have been".
Whatever his reasoning, those two expressions do not usually have the same meaning, but in the context, it really seems he meant the latter.