I think the idea behind using ‘reasonable’ is to eliminate the false promise of ‘ultimate security’ - As that is simply not exist.
Even ‘security’ alone is not a well defined term, but a process to address your threath model. As that should describe your goals and the things you want to ‘protect’ from different kind of threat actors.
And then you must understand what can Qubes OS can give you to help about this. As it is ‘just’ an operating system running on a consumer hardware, it is inevitable to trust your hardware. And this out of scope for Qubes OS - or any other OS out there.
As I would describe what Qubes can gives me:
Qubes OS allows you to be the weakest link - even if you are a highly skilled IT Security professional.
So it is reasonable secure, as there is no ultimate security. And because it is provides you the best available and feasible soultion to address a lot of security concers related to a desktop computer - but surely not all of them.
And just as a side note:
I’m using Qubes OS as my primary desktop since the very begginning of it’s existence. During that 10+ years I triend and evaluated every single ‘competitors’ out there… But I’m still believe that this is the best what you can get if you want a reasonably secure desktop working evnironment. 