Regarding the transcendental determination of the idea of 'everything' - e.g. 'the universe' - all predicates are possible to it: i.e. "all things are possible". Once existence comes into play specific possibilities are crystalised and mutually exclusive ones disappear - they are no longer possibilities.
This transcendental concept is laid out in Immanuel Kant's Critique of Pure Reason:-
Ch. III. Section ii. Of the Transcendental Ideal (Prototypon Trancendentale).
If a transcendental substratum lies at the foundation of the complete
determination of things - a substratum which is to form the fund from
which all possible predicates of things are to be supplied, this
substratum cannot be anything else than the idea of a sum-total of
reality (omnitudo realitatis). In this view, negations are nothing but
limitations - a term which could not, with propriety, be applied to
them, if the unlimited (the all) did not form the true basis of our
This conception of a sum-total of reality is the conception of a thing
in itself, regarded as completely determined; and the conception of an
ens realissimum is the conception of an individual being, inasmuch as
it is determined by that predicate of all possible contradictory
predicates, which indicates and belongs to being. It is, therefore, a
transcendental ideal which forms the basis of the complete
determination of everything that exists, and is the highest material
condition of its possibility - a condition on which must rest the
cogitation of all objects with respect to their content. Nay, more,
this ideal is the only proper ideal of which the human mind is
capable; because in this case alone a general conception of a thing is
completely determined by and through itself, and cognized as the
representation of an individuum.
That is to say, a thing - in our case 'everything', or 'the universe' - notionally has every possibility available to it. Once actualised, the possibilities become limited. If the universe had the possibility of not existing and the eventuality was that it didn't, then the situation would be that "all things are not possible". The possibilities of being possible and not being possible wouldn't exist simultaneously.