No it doesn't go without saying, specially because it means the translations are extremely biased towards showing a positive view of the texts and towards translations likely to reinforce their existing believes.
That's certainly not true of reputable translations - you can easily verify this by just glancing at something widely available like e.g. an annotated version of the NIV - the scholarship and scrupulous attention to detail is evident.
Does that sentence say that Smith is the best candidate? No, it says that Johnson thinks that.
Would it be better if it were reworded to “…were motivated by a desire to seat who he thought was the best candidate as President”? No, that would just be redundant.
Does the original sentence imply that Johnson holds an unbiased, objective opinion of Smith? Of course not. If anything, it implies the opposite!