"Ummm...The slide hold-open is also the safety???

"
The safety lever can be used as a "slide hold-open", but it's actual dual-purpose roll is to position the slide for disassembly.
Note, up is "safe", down is "fire". So in the fire position, would not function as a last round fired "slide hold-open".
The design is nothing more than a copy of the Browning 1906 design. However, the "Royal" is internal hammer fired, whereas the Browning 1906 is striker fired.