When people say 600cc is too much for beginners, they mean 600cc “supersport”.
FZ6 (aluminum frame) and FZ6R (steel frame) both have handlebars which give them a much more upright, street-oriented riding position. In this way they’re more like a standard bike, like a naked SV650 or a faired Ninja 650R, which are more ok for beginners. 600cc supersports like R6/GSXR600/CBR600/ZX6R, on the other hand, have low clip-ons for a crouched over riding position. More than the peaky engine, the low clip-on and the bent over ergonomics (unnatural until you’ve learned how to adapt to it) make a 600cc supersport uncomfortable, much more difficult to learn basic controls on for a beginner, not as easy to balance/turn at low speed so less suitable for street riding.
You can say FZ6 and FZ6R’s inline-4 engines are “detuned” (less peak HP) from R6, or you can say they are made less peaky, less track focused, and more street friendly. They’re still plenty powerful if you rev them up, but they’re actually less torquey than a SV650’s v-twin at lower engine rpm. The character of the engines are different. Inline-4s feel a little like a turbo 4-cylinder car – you need to rev it a bit. A SV650 feels more like a strong V6 – more punchy off the line.