It talks about how their lengths changed, but the very fact that they had lengths before 45 BC is a strong indication that yeah, they existed before then.

The important thing Julius Caesar did was standardise it so that instead of each year being far too short unless the Pontifex Maximus added in extra days manually* it was a regular 365 days with 366 every 4 years. Incredibly close to our current arrangement, drifting from the truth by less than 1 day every century.

The less important but more obvious impact he had on the calendar was that he and his adopted son lent their names to the months of July and August, which is where the common myth comes from that those months were added to honour Julius and Augustus Caesar. The reality is that those months were just renamed versions of already-existing (and already incorrectly-numbered) months.

* conveniently, Caesar himself was elected PM early on in his political career, and it was a lifetime appointment. He was able to use this to his advantage later in life as he was more aware than his military opponents that he had not done that manual shifting in quite some time, and thus the weather was going to be weeks apart from what you’d expect based on the date. So he was able to manoeuvre when they thought it impossible…or vice versa, I forget the details.