Was it presumption for Sts. John and Paul to write that they were heaven-bound?
The short answer
No. We lack absolute assurance of salvation but have a moral assurance: we will be saved if we die in a state of grace, free of unforgiven mortal sin. That explains the apostles' confidence. Presumption is something else.
Moral assurance
We do not have absolute assurance of salvation, but we do have moral assurance: salvation is ours provided we die in a state of grace, without unforgiven mortal sin. This is the confidence with which Paul and John write.
What presumption is
Presumption expects salvation either by one's own abilities or from God without one's own cooperation. The Catechism describes two kinds: presuming on one's own capacities, hoping to save oneself without help from above, or presuming on God's power or mercy, hoping for forgiveness without conversion and glory without merit (CCC 2092).