Am I an asshole for telling someone they were lying about people they know who died from the covid vaccine?

I saw an anti vax post on Facebook, and one of the comments said that the vaccines cause myocarditis. I said that myocarditis from the vaccine is rare, and you're more likely to develop it following a covid infection which is true. Their response was "i know 5 people who died from myocarditis caused by the vaccine" im wiling to bet all my savings that's not true, so I said "I highly doubt that". Is that insensitive even though they are most likely lying to make an argument?

Am I an asshole for telling someone they were lying about people they know who died from the covid vaccine?
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