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That's not the case. SIM cards hold the permanent key for authentication and perform key derivation. Mobile data doesn't pass the SIM card; it does not perform the encryption and decryption.

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Good point--I tend to forget that. The rather vague article seems to indicate the actual SMS content is being sent to the SIM, though. Why is that?

Dumb/feature phones saved SMS messages to the SIM card as simple cards have a limited amount of memory that is dedicated to a crude phonebook and SMS store. Smartphones and smarter feature phones (can) use their own storage for that. You could disable/enable the phonebook/save to SIM features on feature phones and early smartphones.

(I'm talking about win CE and symbian phones being early smartphones here)

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