I don’t think Nagarjuna is “a scholar of early Buddhist texts”.
By his time (AD2-3), the Theravada & Sarvastivada’s Abhidhammas were no yet complete, and a number of Mahayana sutras were just made up.
In order to make people accept those Mahayana sutras, Nagarjuna wrote several theses to demonstrate that “the Mahayana sutras were also said by the Buddha, because their opinions could be found in the agamas”.
So for proofing the orthodoxy of Mahayana sutras, you may not quote from Mahayana sutras themselves.
You really need some old and accepted suttas to support the not-yet-accepted Mahayana sutras.
And Nagarjuna was using the Advaita Vedanta from Brahmanism/Hinduism to distort Buddha’s words.
So as the most important founder of Mahayana Buddhism, Nagarjuna was called the “Second Buddha”.
But he was not the only one for this title, Vasubandhu (AD5-6) and Tsongkhapa (1357-1419) were also called the “Second Buddha”.
Maybe they should be the 3rd & 4th Buddha?
But considering Mahayana has so many Buddhas, it doesn’t matter.
And Nagarjuna was not the only one to certify Mahayana sutras by quoting agamas.
Asanga (AD5-6, Vasubandhu’s elder brother) also quoted a lot of gathas from early Buddhist texts to certify the theories of Vijnaptimātratā School.
Even today, Pings Xiao (萧平实, 平实居士, 1944- ) is also quoting a lot of sutras from agamas to proof his own Vijnaptimātratā equation:
Śūnya(emptiness) = Tathagata-garbha = Alaya-consciousness (the 8th Consciousness)
Of course these people are familiar with early Buddhist texts, but are them scholars of early Buddhist texts?
If someone has the knowledge of zoology, and does a lot of studies, he is a zoologist.
If another guy also has the knowledge of zoology, but uses the knowlege to smuggle animals, he is then an expert animal smuggler, not a zoologist at all!