And your point is?
What is my point?
In the posted article the phrase “China has become the only major buyer of Iranian oil” is the present perfect tense, indicating something that started in the past and continues to the present time.
Since there is zero Iranian oil being transited now and hasn’t been for some time, there are no buyers of that product.
I think the WSJ writer was lazy and cut and pasted something from an earlier article before the U.S. embargo began.