But how can this be true? Humans make irrational decisions all the time. Yes, Alan Greenspan composed the term 'irrational exuberance' in the 1990s, but did Kant not know of tulip mania
It's quite possible for a community of individuals to engage in individually rational behaviour; but which when considered collectively can be irrational; this is one aspect of the Tulip Mania.
Also time is a factor; what is rational in the short-term may be irrational in the long-term; and the reverse is possible too.
And, a man may be rational but his understanding might not be sufficiently penetrating; such that his actions look irrational.
And then there is the obvious factor of information asymmetries.
Not only is the capacity of reason universal, such capacity is rational universally?
The second statement, is simply a restatement of the first: such capacity must refer to the capacity of reason, which is rational - by the usual understanding of what reason and reasoning means; and by stating that it is universal we come back to the first statement.
But this, I think, is not what you mean; it seems that you are arguing why should mental activity which is shared by all thinking beings, be rational for all such beings, and not only now and then, but at all times.
In a sense, it is a principle or an axiom that Kant is stating, that can be critiqued as you are doing, but which one accepts to see where he is going in his overall argument.
One might justify it, by saying were we to understand every thought a rational being has thought, and every desire he has felt, and every sensation that he has experienced then we can understand, or make sense of why and how he acts.
