This is apparently a widely propogated claim, but a big reason for why this happened was due to the state of Kerala having one of the biggest and wealthiest diaspora communities who send back enormous amounts of remittance money which to this day keeps the entire state's economy afloat and allows the local government and local communities to afford various social programs. The entire model collapses if money from diaspora in the West and the Arabian gulf states stops flowing in to prop it up. It's also historically had a head start compared to the rest of the country even since before independence since it was not a directly governed colony but a client princely state of the British Empire.

Claiming that Indian socialism is responsible for their low poverty is also a little disingenuous because the neoliberal party has been in charge in many spells (nearly half of the time I think, including recently) throughout the state's history, so not really indicative of an enormous socialist tradition.