This is a principle based explanation of nature of establishing Suhoor time, that considering the nature of ‘talu-e-fajar’, there is room for subjective conclusion and in deciding suhoor time, there can be slight differences between different sects and between different religious groups as well. However, we should follow the time that we genuinely think is the right time. The exception is only limited to genuine cases where someone is already eating something and in-between that eating, the time that he thinks is the right time for Suhoor already ticked the clock. The prophet (SWS) only allowed such a person to finish their meal and not leave their meal in-between. This should not be considered as a general principle. If someone adopts this as a habit of going against what he truly believes to be the right time of Suhoor, is an attitude of istakbaar and we all should be mindful of that.
Ghamidi Sahab has also explained this very categorically in the following links:
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