Well, he does keep going out there and saying wild things. Tomic may still be young, but he's certainly not young in tennis; he's spent nearly 4 full years inside top 500, so I guess he should have some idea how things work by now. His goal for 2012 was finishing in top 10 or in top 20 at the worst. Now that he failed to finish in top 50, his new goal is the be the GOAT. It's almost funny how everything with him is backwards and upside-down.
1. there's a difference between what one really thinks and what one says in the media : I don't think he believes one word of that "goal", it's just not coherent admitting Goffin's superiority one day and thinking "I will be the best ever" another day, do you think he really believed both ? ;
2. it's just the final sentence in his interview, and it's a copy-paste of his father's words, his father being known for wanting what is said about his son in the media and being the one who has pushced his ambition.
Djokovic managed not to repeat his father's silly thoughts in the media, not everybody can manage the same, as I said Djokovic looks like a very clever guy (besides Tomic's father's presence is much more important than Novak's in his tennis life)