My ex employer has served me legal notice for posting reviews on web

I joined ABC 8 months ago. I used to work there in the field of online marketing. I was previously in sales but I learned digital marketing during that time and earned prestigious certificates in that field. Post that I joined ABC, but to secure a position there I mentioned in my CV that I had worked as a digital marketer in my previous organization. Further, I had taken a loan from my previous organization and they were not relieving me without the repayment of my loan, which I paid by obtaining a personal loan.But before obtaining the loan I had once requested the CEO of ABC to provide me with the remaining loan amount so that I could be relieved from my previous organization, which he refused. Later I joined ABC, and I found the environment quite unprofessional. I left it after 6 days without notice. Now they have sent me a legal notice for defamation and fraud as I posted a review online of ABC. Further, they are blaming that I tried to secure the loan via fraudulent means etc.