As reflected in the screenshot provided, it would "change object's storage class to Glacier after 10 days" and later "delete the object (add delete marker) after 15 days".
The part about "Glacier has 90 days minimum storage duration" applies to pricing. Though you are would be able to delete the object from Glacier in 15 days, you would end up paying for the remaining days of minimum storage duration. This info is found in the fine print in the S3 pricing page and the question "How am I charged for deleting objects from Amazon S3 Glacier Flexible Retrieval that are less than 3 months old?" in S3 FAQ.
Thank you Syd. It turns out "Transition takes precedence over creation of delete markers. Initially it changes storage class to "Glacier". Later it deletes the object (adds DM in case of versioned bucket).
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