Isn't "reduces temperatures by 15 Kelvin" the same as "reduces temperatures by 15 Celsius"?

Yes, Kelvin is only a linear offset from Celsius. (273.15 for anyone who doesn’t already know).

It’s a little bit funny/coy to use it mixed with Celsius.

reduces temperatures by at least 15 Kelvin == the same as reduces temperatures by at least 15 Celcius.

It 'reduces it by' ... not reduces it TO