:-) In the demos that I've seen they even say...
"Don't do this."
I think it's to emphasize that while it can mimic the native environment of either platform Flutter also has the ability render non-native custom controls to either platform.
This is the equivalent to saying that a cigarette boat could get a couple shots off in a fight with a battleship because of it's maneuverability and speed.
Well yes that may be true.
However, it's more likely a battleship would sink a cigarette boat with a precision guided weapon a second after it becomes visible on the horizon.
Knowing the number in your situation doesn't change much in your situation. You still won't provide the first number.
However, since you know where the first number will be (within 5-10K) depending on your presentation up to this point.
Either:
A. You will provide a reason or reasons you are more valuable than that number (in value to the company, which they will accept or reject)
B. You will show you are more experienced than someone who would be a good fit for this position
C. You will accept their assessment of an offer in that range.
You should research the market value of this position it won't help your negotiation (I wouldn't recommend trying to convince someone they are paying their people too little) but it will tell you whether their offer is acceptable.
I think it's to emphasize that while it can mimic the native environment of either platform Flutter also has the ability render non-native custom controls to either platform.