WhatsApp marketing firm are dim a dozen who will help send messages via API. It's banks, food delivery, shipping companies... All send unsolicited WhatsApp messages and i think the government just uses one of the marketing agencies for it. Also, it's not against the meta rules as they aren't promoting any party but government communication.
According to some stats, India has 487 million WA users. Sending these many messages from a single account should have raised a red flag.
I can understand the sending of mass messages in a national emergency, but this one is for political gain.
The ruling party in India is using government missionaries to spam its citizens for their electoral benefits. How could WhatsApp allow this?