Is Sumokoin able to repel attacks like Monero V's that threaten Monero's anonymity?
Well according to this
https://btcmanager.com/monerov-trap-laid-monero-users/(end of the article at the "Conclusion" part)
As for a concrete solution to chain split attacks, a higher minimum enforced ring size could be the answer. Ehrenhofer, along with Brandon Goodell, recommends a higher ring size and increasing this figure from five to eight, but comes with the cost of increasing fees and transaction sizes, For a modest increase in fees and transaction size, we can be much more assured that Moneros ring signatures are prepared for large chain split attacks. Goodell proposed a range between eight and 16. With a ring signature of eight, every transaction will have eight signatures instead of five and eight possible inputs for one actual input spent, providing greater obfuscation. The attractiveness of this solution is that it is easily implemented.Sumokoin has by default and since day one,
12 mixins which means a ringsize value of 13, also Sumokoin has a young chain with still small fees while bulletproofs implementation will keep the chain and fees at the current levels so this high ring value doesnt have any serious negative effects on its chain size and fees.
So I guess the answer is no, I think Sumokoin is secure from such kind of issues