[Edit: I mean, he's complained that it's unjust and that billionaires like him should be paying their fair share.]
https://www.businesswire.com/news/home/20161010005859/en/Tax...
Trump paid $750 total, which is not 15% of anything. He could have easily paid $0, as he did before becoming POTUS. His team picked a tiny number above zero starting in 2016 when he was elected. This is completely and totally unrelated to the capital gains tax rate.
Almost certainly, Trump used these loopholes to minimize his taxes. I assume in his case, he had a lot of losses that he was able to use to bring his over all income down well below what you or I report in a given year.
Unless Trump did something illegal (entirely possible), my point is related and not distracting.
(This is a joke, to be clear.)
He donates to charity 100% of his US government salary. I'm wondering how he even owed $750 on federal taxes.