However, from skimming the text, I would have to agree with the sibling commentator, that pointed to the reconstructed Proto-Dravidian language, rather than Tamil. The book you've shared suggests links of the IVC with Proto-Dravidian, not Tamil directly. Tamil as a distinct language is not attested before the 3rd Century BCE, making it a contemporary of the Prakrits in the North, around the time of Ashoka. The Sangam Literature period starts from the 1st Century BCE, roughly contemporaneous with the advent of Indo-Greeks in the North. Tamil =/= Proto-Dravidian, which would indeed have been around at the time of the IVC. Whether the IVC had contact with Proto-Dravidian or were Proto-Dravidian themselves is still an open question. Even the preliminary Ancient DNA evidence from the IVC does not settle this matter conclusively. (I'm a Geneticist, not a linguist)
However, from skimming the text, I would have to agree with the sibling commentator, that pointed to the reconstructed Proto-Dravidian language, rather than Tamil. The book you've shared suggests links of the IVC with Proto-Dravidian, not Tamil directly. Tamil as a distinct language is not attested before the 3rd Century BCE, making it a contemporary of the Prakrits in the North, around the time of Ashoka. The Sangam Literature period starts from the 1st Century BCE, roughly contemporaneous with the advent of Indo-Greeks in the North. Tamil =/= Proto-Dravidian, which would indeed have been around at the time of the IVC. Whether the IVC had contact with Proto-Dravidian or were Proto-Dravidian themselves is still an open question. Even the preliminary Ancient DNA evidence from the IVC does not settle this matter conclusively. (I'm a Geneticist, not a linguist)