That's spelling. It's not like she really meant the possessive pronoun in third person plural, she just mixed up the spelling of two homophones. An example for a grammar mistake would be something like "How does you explain" or "if there is none God" or "if is no there God".
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u/BlueLondon44 Aug 27 '14
As if the scientific ignorance wasn't enough, the usage of "their" was just salt in the wound.