The author hypothesizes that the Chicxulub asteroid strike wasn’t responsible for most dinosaurs going extinct. Why? Because even a major asteroid strike doesn’t have worldwide consequences. What's more, there are even larger asteroid craters out there, from strikes that were accompanied by no known extinctions whatsoever.

The author is making a phenomenon-hypothesis argument, saying that a particular hypothesis (that the Chicxulub asteroid didn't wipe out the dinosaurs) best explains observed phenomena (the extinction, but also what we know about asteroid impacts). To weaken, we can look for an alternative explanation. We can also search for overlooked considerations in order to attack the author's assumptions.

The author's hypothesis is supported with a claim about the low potential for worldwide impact from an asteroid strike, and an analogy to other major asteroid strikes. Both of these require assuming something about the Chicxulub strike, and we can weaken by attacking those assumptions.

For the limited worldwide impact of an asteroid strike to be relevant, the author must assume that the dinosaur population was distributed worldwide. If dinosaurs were all concentrated near Chicxulub, a local impact could be sufficient to wipe them out.

For other asteroid strikes to provide useful evidence, they must have been relevantly similar to the Chicxulub strike. In other words, the author assumes that there was nothing meaningfully distinct about the Chicxulub asteroid, or the conditions at the time it struck.