You have answered your own question. The Mundaka Upanishad quote pertains to the formal study and transfer of this knowledge in the textual tradition. But anyone is capable of realizing the Brahman even without formally being part of the formal guru-shishya parampara. There are so many examples in the scriptures.
Satyakama Jabala himself realized Brahman before his guru formally taught him. See Chandogya Upanishad 4.9:
प्राप हाचार्यकुलं तमाचार्योऽभ्युवाद सत्यकाम३ इति भगव इति ह प्रतिशुश्राव ।१
He reached the gurukulam. The Acharya called him, "Satyakama!" and he replied, "Sir!"
ब्रह्मविदिव वै सोम्य भासि को नु त्वानुशशासेत्यन्ये मनुष्येभ्य इति ह प्रतिजज्ञे भगवाँस्त्वेव मे कामे ब्रूयात् ।२
"You shine like a knower of Brahman! Who taught you?" He replied, "Somebody other than humans. I would like Sir to teach me as well."
श्रुतं ह्येव भगवद्दृशेभ्य आचार्याद्धैव विद्या विदिता साधिष्ठं प्रापतीति तस्मै हैतदेवोवाचात्र ह न किंचन वीयायेति वीयायेति ।३
"I have heard from people like yourself that only knowledge that is received from a teacher becomes effective." Hence, he taught him the very same knowledge (that he had already gained), and there was nothing omitted.
But in the beginning of this episode, Satyakama approaches the Acharya without even knowing who his father is, i.e. he doesn't know his family lineage i.e. whether he was born from a traivarnika father.
So the difference is between going to a formal school or college to earn a degree versus learning the same skills while doing a job (self-taught). The end result is the same as far as application goes, but the degree has a formal recognition.
Similarly, in the life of Adi Shankara, there are two incidents. One is where a so-called Chandala is walking in front of Shankara, and when told to move away, he asks how he can move the Atman which is everywhere. Shankara immediately realized that this was a great realized person and fell at his feet. Another incident is of Giri, who was self-realized but was mentally slow and was slow at learning the scriptures. Under the grace of Shankara he became known as Totakacharya.
In recent times, we have the example of Ramana Maharshi, who never formally studied any scriptures.
EDIT: To answer the comments which say that the above are only exceptions.
Please read carefully what I have written at the beginning. The capability to realize Brahman is innate in everyone, regardless of caste or guru or any formal education.
Adi Shankara himself says this in his Brahmasutra Bhashya on Sutra 1.1.1 (अथातो ब्रह्मजिज्ञासा). In listing the prerequisites for realization of Brahman, he says:
नित्यानित्यवस्तुविवेकः इहामुत्रभोगविरागः, शमदमादिसाधनसंपत्, मुमुक्षुत्वं च । तेषु हि सत्सु प्रागपि धर्मजिज्ञासायाः ऊर्ध्वं च, शक्यते ब्रह्म जिज्ञासितुं ज्ञातुं च, न विपर्यये ।
Discrimination between eternal and temporary things, disinterest in the enjoyments of this or other worlds, the possession of qualities like Shama (mental peace) and Dama (physical calmness or non-hankering of senses)*, and the intense desire for liberation. In the case where these are present, it is possible to learn about and realize Brahman even without involvement in yajnas and scriptures or even after this. But not in the case where these are not present.
*शमदमादिसाधनसंपत् -- The full list is शम (settled mind), दम (settled senses), उपरति (cessation of sensual enjoyment), तितिक्षा (patience).
None of the above are dependent on any external preparation. It all comes from within oneself. Even if a guru hammers it on the head for decades, if the person doesn't get it within himself, then it's not going to happen. Only in this sense, Brahmajnanis are exceptional. Not by whether he is a Shudra or not.
Even studying all the scriptures formally may not imbibe a person with the above qualities. As seen in Chandogya Upanishad chapter 7, Narada lists all the Vedas and other scriptures he has studied, but still has not self-realized Brahman.
