I do agree that there's an implicit intention in the rules that the operative's guesses are related to the spymaster's hints, but I don't think that relation is ever said explicitly.
A common pattern in our play groups is when the other team is just 1 away from winning and you cannot find a clue that covers all your remaining words, you come up with the best clue you can and say "99" so that it's clear "The number is irrelevant to the clue here, just keep guessing until you win or you miss."
Edit: I'm wrong. From the rules: "If 0 is the number, the usual limit on guesses does not apply. Field operatives can guess as many words as they want. They still must guess at least one word."
Edit2: And instead of 99, my play groups should have been using "unlimited." Again from the rules: "Sometimes you may have multiple unguessed words related to clues from the previous rounds. If you want your team to guess more than one of them, you may say unlimited instead of a number. For example feathers: unlimited."