I know this maybe a dumb question, but in this verse (19:26), it says:

"So eat and drink and refresh your eye; and if you meet any mortal, say: ‘Verily I have vowed a fast to the Beneficent (God), so never shall I speak today with any human being’.

I read the tafsir, and it said that, Prophet Jesus (PBUH) spoke and defended her chastity. But if she’s doing a silence fast, why does this verse say that if she meets a mortal she’ll say that she’s fasting. I know that Prophet Jesus (PBUH) talked and Lady Mariam (PBUH) didn’t but my question is why this verse says so.

She signaled to them that she is fasting through hand gestures. This was a common fast that was done before Islam. Zakariya also did it after God informed him he will have a child.