Please read the following link:http://buddhasutra.com/files/mahapadana_sutta.htm
Even Pali sutta itself acknowledge that Buddha has reached enlightenment.
Both schools agree that Buddha Sidharta has become a Buddha since donkey years before he manifest himself in India.
One thing which is very confusing from Theravada tradition is some of them say Buddha has not reached enlightenment, but their own sutta tell them, Buddha has reached enlightenment. May be this group of people haven't read the above sutta, so they have that mindset.
Mahapadana Sutta - PALI SUTTA
The Great Discourse on the Lineage
Thus have I heard. Once the Lord was staying at Savatthi, in Anathapindika’s park in the Jeta Grove, in the Kareri hutment. And among a number of monks who had gathered together after their meal, after the alms-round, sitting in the Kareri pavilion, there arose a serious discussion on former lives, as they said, "This is how it was in a former life," or "That is how it was."
And the Lord, with purified divine-ear faculty surpassing the powers of humans, heard what they were talking about. Getting up from his seat, he went to the Kareri pavilion, sat down on the prepared seat, and said, "Monks, what was your conversation as you sat together? What discussion did I interrupt?" and they told him.
"And so it is, monks, that by his penetration of the Fundamentals of Dhamma, the Tathágata remembers the past Buddhas who have attained final Nibbána, cutting through multiplicity, blazing a trail, have exhausted the round, have passed by all suffering; he recalls their births, their names, their clan, their life span, their twin disciples, their assemblies of disciples: "These Blessed Lords were born Thus, were called thus, thus was their clan, thus was their morality, their Dhamma, their wisdom, their dwelling, thus was their liberation.""
Thus the Lord spoke, and the monks, delighted, rejoiced at his words.