Sounds like a development of opportunity. Article says the movie was facing financial issues and Kartik offered to forgo his fee. So the producers decided to credit him as a producer. It doesn’t sound like he took on any creative or logistic responsibility of a producer.
i think this is negotiation, after south remakes which have already released dubbed versions in hindi (like jersey, vikram vedha?) flopped the producers wanted to go direct OTT route for the hindi version of this, but there was news that kartik was adamant about the theater release, the forgoing of fee for producer credit might be the outcome (he taking the risk for insisting on theater release)