I don't agree at all. They do have something in common: they're stack-based. But what they don't have in common is everything else. RPL is a dynamically-typed, bounds-checked, memory-safe, garbage-collected language similar to PostScript or Python. Forth is an untyped, unsafe language with arbitrary compile-time compilation and without even a heap, traditionally. It's the minimal veneer on top of assembly language to give it arbitrary compile-time metaprogramming and nested expressions and control structures. RPL doesn't even have compile-time metaprogramming at all.

These two languages represent diametrically opposed approaches to programming language design.