If it was a loan, and you were to die, the money would have to be repaid from your estate. That doesn't apply though.
Similarly, if it were a loan and you were to move into a care home, you'd still have to repay it. You won't though.
If it were a loan, someone who starts paying fuel bills after such loans were made (e.g. a young person who moves out of their parents' house and into their own home at the end of this year) wouldn't have the 'repayments' added to their own bills. They will though.
It's a grant but one where the Government will get their money back through a levy on fuel bills later on (irrespective of whether the people paying those bills were actually those in receipt of the grants).
https://www.gov.uk/government/news/energy-bills-support-scheme-explainer