That's a very negative presumptions.
How about the oldest attestation of Indo-European language or the long extinct language Hittite who once lived in Bronze age Anatolian Steppe? The language is attested in cuneiform, in records dating from the 17th to the 13th centuries BCE.
Hittite people created an empire centred on Hattusa, and also around northern Levant and Upper Mesopotamia [1].
[1] Hittite language:
The Hittites adopted the Sumerian form of writing; they did not bring a writing system with them from the Volga. Neither did other Indo-European groups have writing, which is why Hittite is, as you say, the oldest attested Indo-European language.
That Indic language was the language of some group of people who at some point in time, perhaps after a war victory, had become the main members of the elites who ruled Mitanni, a Southern neighbor of the Hittites, located mostly in present Syria, where most inhabitants were speaking Hurrian, a non-Indo-European language.
Those Indic-speaking people were renowned as expert horse trainers, so the quotes from their language were encountered in Hittite documents about horse training.
Most known data is consistent with an older migration towards South Asia of the people speaking Indic languages, who had gone both towards East, reaching India, and towards West, reaching as far as Syria, where they entered in contact with the Hittites and other related populations, who had migrated towards South at an even earlier date and through a different path, reaching present Turkey.
The Indic migration has been followed much later by a migration on the same path of people speaking the closely related Iranian languages, who have reached the present territories of Iran, Afghanistan, Tadjikistan, forming the ancient Persian empires, after various conquests.
The people whom we now name Hittites used another name for themselves, and they called Hittites a non-Indo-European population, who were the former inhabitants of the territory ruled by what we call Hittites.
It is likely that various kinds of "proto-writing" have been independently invented in a lot of places, but very few of them have evolved into writing systems.
"Proto-writing" is just a set of graphic symbols that are used to designate various things. Such a set of symbols can be used e.g. to write an inventory, to tag things to show ownership or purpose, to show on a map what can be found in certain places, and so on.
"Proto-writing" cannot be used to write human speech. All systems of "proto-writing" that have evolved into writing systems have done that by reinterpreting a part of the graphic symbols, or sometimes even all of them, to no longer be the names of some things, but to have a phonetic meaning, i.e. to represent some sounds of human speech (syllables in almost all cases), allowing thus the writing of the more abstract components of the speech, like various grammatical markers.
Therefore for a system of "proto-writing", it does not make sense to ask which is the language that has been written with it, because there exists no such language.
The only kind of information that can be known about a system of proto-writing is which is the thing denoted by each symbol. Even when the meanings of all symbols are known, that does not offer any information about the language used by those who have invented and used that system of proto-writing.
For now, there is no evidence that the Indus script was a writing system, because only very short strings of symbols have been preserved. It could have been a writing system, because by that time other writing systems already existed not far away, which could have inspired them, or it could have been just a proto-writing system, which would give no clue about the language of its users.
