When may the two Sunnah rak'ahs of Fajr still be performed after the congregation has started under the exception taught here?

Prohibited and Disliked Times of Prayer

• Whenever the person prefers Sunnah over congregation.
• When they expect to complete them separately and still catch part of the Fard congregation.
• Only if the congregation is expected to last a very long time.
• Never under any circumstance.