The sentence has a logical problem, that's why it looks wrong to you.
Columbus gazed at the land he had come/travelled so far to seek: he (had seen) nothing but ocean for five long weeks.
The word from is throwing the sentence out of kilter.
<em>Columbus would not with his own eyes or an instrument of the day, be able to see both the land he saw and the land he had come from or vice versa.</em>
Past perfect describes something that happened in the past before something else.
Prior before seeing the land he sought he had seen nothing but ocean.
1-Columbus gazed out at the land he saw before him, 2- he had previously seen nothing but ocean.
Textbooks can complicate sentences. Often a simple rewrite into daily speaking sentences clarifies things.
1-What did columbus see 2-after sailing for so long 1-he saw land.
1-Columbus gazed out at the land he saw 2- he had previously seen nothing but land.
"Columbus gazed out at the land he saw before him, he had previously seen nothing but ocean".