while it says he built some rockets at home, we never actually see how they looked and whether we're talking about models or actual funcitonal rockets. Since nobody can create a space rocket that isn't like 40 meters tall at least, it means at most he made model rockets and understood the functions of a rocket really well.
Gru on the other hand made a fully functional rocket at around 5 years of age. Most humans only became fully concious of their actions a year before, meanwhile at that development stage he managed to achieve a fully functional rocket.
Also to note, this happened right after the moon landing, meaning that Gru did this at around 1970 at which point there was far less information readily available about rocket science, compared to the time at which Taylor lives, when the internet was already a thing and far more information was available to the common person.