Where in the book does it say that they didn’t actually make it to the center of the earth?

Axel and his companions continue along Saknussemm’s path, but are stopped by a huge boulder that must have lodged in the passageway sometime between his journey and their own. Now flush with zeal for the journey, Axel suggests using firepower to blow an opening. The explorers set this plan in motion and wait on their raft.

After the explosion occurs, Axel, his uncle, and Hans realize that they've created a disruption. The entire sea goes rushing through the aperture and the three men are carried wildly along on the waves. THe men get washed away which implies that they never got to the final center.