Such an ideal state must be regarded as possible, because it is a necessary postulate of reason.

From this point of view it may be seen that Kant's speculation on universal history is really a discussion whether the ideal state, which is required as a subjective postulate in the interest of ethics, is likely to be realised objectively.
Now, Kant does not assert that because our moral reason must assume the possibility of this hypothetical goal civilisation is therefore moving towards it.

That would be a fallacy into which he was incapable of falling.

Civilisation is a phenomenon, and anything we know about it can only be inferred from experience.

His argument is that there are actual indications of progress in this desirable direction.