Lawyer here.
I don’t think their case will stand in the court.
They are just threatening you to remove the negative review.
There is a judgement from Madras Highcourt about such cases.
Reviews cannot lead to defamation.
Have a look at link below:
https://www.livelaw.in/high-court/madras-high-court/madras-high-court-right-to-free-speech-includes-right-to-share-reviews-on-online-platform-237667