Great question! First, responding to a broader concern you raise:
Any clarification on this question stem (author implies if theory __ was true, X must be true) would be helpful.
If you see a question asking (whether on logical reasoning or reading comprehension) which of the answer choices is "implied" by the passage/stimulus, then you should take that as indicating that it is a must be true question type.
To your more specific question about this passage and question, the question asks whether, "if the theory described in the third paragraph is true, which one of the following would have been true of iridium in the atmosphere at the end of the Cretaceous period?," as described in the third paragraph. You correctly cite to the relevant portion of the passage in pointing to the sentence from line 39-47:
Although iridium is extremely rare on the Earth's surface, the lower regions of the Earth's mantle have roughly the same composition as meteorites and contain large amounts of iridium, which in the case of a diapir eruption would probably be emitted as iridium hexafluoride, a gas that would disperse more uniformly in the atmosphere than the iridium-containing matter thrown out from a meteorite impact.
In speaking to the broader function of that paragraph, the first sentence indicates that it's a continued expiation of the "volcanic-eruption theory." Answer choice (D) states that "It would have been more uniformly dispersed than iridium whose source had been the impact of a meteorite on the Earth's surface." This is directly supported by the third paragraph. Inasmuch as you have a concern that an answer choice would be wrong because the question asks for what is "implied," and the answer choice you're considering is true but explicit rather than implied, that's almost certainly the right answer. Here, the language in the third paragraph is in the subjunctive ("would probably be emitted
as iridium hexafluoride..."). In other words, if
the theory described in that paragraph were true, then X, Y, Z would be expected to occur. If
the volcanic-eruption theory were true, then one would expect the more uniformly dispersing iridium hexafluoride.
Again, in general, if you see a question stem asking what is "implied" by the passage/stimulus, you should treat that as a must be true question, but hope that also clarifies this particular question!