Do comets contain any significant amounts of iridium? I have Googled "iridium content of comets" and can't find anything that suggests comets contain iridium in any detectable quantities. Yet, whenever I read an article about the Chicxulub impactor, the article invariably says something to the effect that it may have been created by an asteroid or a comet. But Alvarez and company first offered the impact hypothesis for the K-T extinction by observing an iridium layer in global sedimentary deposits, which could only have come from an asteroid impact, since iridium is extremely rare in the Earth's crust. Later the Chicxulub structure was identified as the probable site of the asteroid impact. While a comet could have caused the Chicxulub crater, it could not have been responsible for the global iridium layer. So why do science writers offer the comet alternative? If Chicxulub was created by a comet, then another site must have been the source of the iridium layer.
I also googled "iridium content of comets", and the first result was https://news.dartmouth.edu/news/2013/04/dartmouth-researchers-say-comet-killed-dinosaurs

Now there is currently no consensus on the nature of the Chicxulub impactor. The observations of iridium and osmium suggest an asteroid. A minority opinion is that a comet may be responsible. The authors of the article in the linked news report suggest that measurements of iridium concentrations from ocean cores may be erroneously high, due to sorting in marine sediments. If these cores are excluded, the amount of iridium measured is lower, and consistent with some models of the composition of comets. 

So, comets do contain some iridium. The amount of iridium is not certain. Some models of comet nuclei contain more dust and less ice, and so have more iridium. The exact amount of iridium in the KT boundary layer is also uncertain. The low end of estimates would be consistent with a cometary impact. The nature of the impactor is not yet settled science, but the balance of opinion currently favours a stony asteroid.