As a very rough rule of thumb, if you have spent over 50% of the last 12 months or the last 24 months in Thailand on Visa Exempt and Tourist Visas, you run the risk of being denied entry.
In this case, I strongly believe the 15 days is completely irrelevant. You would need to look at the denied person's travel history over the last 12 - 24 months.
I was denied entry in 2019 having spent a month in the UK. The reason was I was spending around 75% of my time in Thailand on METV's. (Of course, they do not overtly state that, you have to figure it out.)