य एकश्चर्षणीनां वसूनमिरज्यति |इन्द्र पञ्च क्षितिनाम् || RV 1.7.9
Indra who alone rules over men , over riches , and over five (classes)
of the dwellers on earth. (H.H.Wilson)
The the sanskrit word used in the mantra for fivefold race is "Pancha kshitinaam - पञ्च क्षितिनाम् " .
Your question asks what this "fivefold race" is referring to.
And yes, your guess is correct about the word Pancha kshitinaam. Here is what H.H.Wilson explains about the word with the commentary of Sayanacharya.
The text has over the five men or classes of men, Pancha kshitinaam. The latter term is explained etymologically as those who are fit for habitations (Nirvasarhanam). The phrase is not of infrequent recurrence, and is usually said to imply the four castes: Brahmanas, Kshatriyas, Vaishyas and Shudras, and Nishadas, barbarians or those who have no caste; [referring], possibly, to the aboriginal races of India, all in a very low stage of civilization, like the Gonds, koles and Bhils of present day.
Here is what Pandit Shree Ram Sharma Acharya in his translation of Rig-Veda is saying . According to him also the word Pancha kshitinaam is used for five classes of men.
But apart from that the word is having another meaning and according to some scholars is used in different contexts than above.
First of all let's see what the Arya Samaj translation is telling us about the meaning of the word.
According to Swami Dayananda Saraswati the word is referring to five earths, which are according to their grades i.e. low, medium, good, very good and best. These are the five planes or the earths or we can say Prithvi lokas graded according to their quality. And Indra is the lord of all these Prithvi lokas alone.
We can read Swami Dayananda's translation here .(Page. No. 122 )
The above theory is also mentioned by another author M. P. Pandit in his book Wisdom of the Veda.
In his translation the author is using the word five planes for Pancha kshitinaam.
य एकश्चर्षणीनां वसूनमिरज्यति |इन्द्र पञ्च क्षितिनाम् || RV 1.7.9 ||
Indra who alone disposes all actions and possessions of the five planes .
This is his explanation.
Indra the power of the divine Mind is the sovereign of the being of man on all its planes of existence. Man is not merely his physical body. Exceeding his physical existence, in his life organisation; above that is his mental consciousness ; higher are the realms of the light of knowledge and still above are the regions of blissful Transcendent, the parardha. Movement in all the five planes of being, the five earths of veda, are controlled and disposed by Indra.
So these Pancha kshitinaam is five planes which are above his physical body which are above each other and each is excellent than the below one are the five earths in vedas.
Conclusion - : So from all the translations above we can say that the word Pancha kshitinaam has two different meanings. One is five classes of people i.e. Brahmanas, Kshatriyas, Vaishyas, Shudras, and Nishadas. And the other one is deeper and associated with our own koshas or sheaths, which are called prithvi lokas in vedas. As five earths, here one earth is human body and these five earth planes are above the physical body i.e life organisation, mental consciousness, light of knowledge and blissful Transcendent.
Arya Samaj Translation.