Agreed that the ablative absolute is not involved here. There's no participle anywhere to be found, let alone one in the ablative case.

"Tutela" means "protection" or "guardianship," not "a protected one," which might be "tutelata." So "I the protected one was safe" would be "tutelata valui."

Rather, "tutela" is in the ablative case (with a long a) in this sentence instead of the nominative (with a short a). Thus, the translation is "by protection was I safe."

(Conceivably, "tutela" could be nominative, but then the sentence would be "I the protection was strong," which doesn't make much sense.")