The Tamil independence movement is a colonial collaboration movement. They are not socialists. They are not a decolonial movement. (Tamils were the colonizers before the british showed up but that is literally ancient history.) That isn't to say the republic government is any good.
The Tamil people were given preferential treatment under the british and so the Tamil leaders wanted reform not revolution to get rid of the british. They demanded 50/50 representation in any socialist government despite having only 12% population. The socialist revolutionaries said that wasn't realistic and so the Tamil leaders pulled their support. The intractable stance of the Tamil leaders destroyed the communist revolution and empowered the reactionary nationalists on the Sinhalese side.
As the minority Tamil do have genuine cause to be fearful of cultural assimilation and the socialist movement should have done more to assuage those fears but the Tamil leaders were also too demanding and by rejecting the socialists and leaning into their benefits under british rule they gave the balance of power to nationalists.
The repression of Tamil language was an overcorrection for the preferential treatment by the british but again that was because the Tamil leaders chose the colonizers over the socialists. If the Tamil leadership sided with the socialists together they could have dealt with the british and then the nationalists. Instead the socialists made deals with the nationalists to end british rule and when the nationalists went for the Tamil the socialists didn't bother standing up for them.
Also Tamil resistance groups were (are?) armed by India and often fund themselves with heroin smuggling. They are not the good guys just like the "Free Tibet" movement or "East Turkestan" of the contras in Nicaragua.
The whole thing is a perfect storm that undermines de-colonialism and socialism. The british set up the pieces like they did all over the world but the Hindu supremacists in India really brought it to another level. Playing ethnic minorities against the revolutionary movement to give reactionaries more power is impressive work even if most of it was just luck.