Can I safely charge a SGS9 device with a 5.2V MacBook Pro 2018 USB-C charger?
According to Wikipedia the USB specification is:


  USB supplies power at 5 V ± 5% to power USB downstream devices. (source)


5% of 5V are 0.25V 

Therefore any USB device have to be able to work correctly as long as the "received" power via USB is within the range of 4.75V to 5.25V.

The Apple USB power supply with 5.2 Volt is within this range.