What do you think of people who say "it's just my opinion" if their claim challenged?

For example, someone makes an argument claiming Earth is flat. Then I would explain why it isn't and question why they think so. They explain and we exchange arguments. Then after a long conversation, they're just going to say it's just their opinion in the end just when they're about to lose credibility.

In a nutshell,
Person 1: Earth is flat because of TikTok or whatever.
Person 2: No it isn't. *Shows millions of studies*
Person 1: That's just my opinion

I'm thinking that tactic getting too common now. Using the word "opinion" to shield themselves wrong being wrong because apparently nobody is right or wrong when giving an opinion. Also a way to disguise themselves for being dismissive. They would never say that if anyone agrees with them.
What do you think of people who say "it's just my opinion" if their claim challenged?
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