A tautology is a logical certainty with no information. A tautology says the same thing twice with different words. Existentialism affirms we gives meaning to our own lives through living those lives. Existentialism is therefore a form of tautology in that the meaning of life is the meaning we give to our lives. The meaning is our meaning, not quite a tautology
The famous assertion “I think therefore I am” contains the clear implication that thinking presupposes existence. The assertion is that some things that exist think. At some deeper level, perhaps, the assertion also implies that existence itself is reified by thinking, which then would be a tautology: that we know we exist by thinking; that the act of thinking is the epitome and essence of existence. At the very least, those who recite the sentence “I think therefore I am” propose the sentence means thinking is evidence of existence. Thinking then supports the supposition that existence is supported by thinking.
Should we accept the premise that thinking somehow reifies existence we have accepted the premise of Rational Existence. Existence is not physical but rather rational. Our models and concepts and words and explanations and definitions are what make our world. Existence cannot be known, however we can know relationships that exist, as well as other aspects and attributes of existence.