From the Berkeley News article:
Richards teamed up with experts in many areas to try to discover faults with his radical idea that the impact triggered the Deccan eruptions, but instead came up with supporting evidence. Paul Renne, a professor in residence in the UC Berkeley Department of Earth and Planetary Science and director of the Berkeley Geochronology Center, re-dated the asteroid impact and mass extinction two years ago and found them essentially simultaneous, but also within approximately 100,000 years of the largest Deccan eruptions, referred to as the Wai subgroup flows, which produced about 70 percent of the lavas that now stretch across the Indian subcontinent from Mumbai to Kolkata.
...Co-authors of the paper, in addition to Richards, Renne, Manga and Sprain, are Walter Alvarez, a UC Berkeley professor emeritus of earth and planetary science and the co-originator of the dinosaur-killing asteroid theory...
From The Atlantic article
Over and over, Keller saw “no evidence of a sudden mass killing.” Instead, she found more proof that the Earth’s fauna grew progressively more distressed starting 300,000 years before the extinction. The forams, for example, gradually shrank, declined in number, and showed less diversity, until only a handful of species remained—results consistent with what many paleontologists have observed for animals on land during the same time.
...Alvarez had set the tone. His numerous scientific exploits—winning the Nobel Prize in Physics, flying alongside the crew that bombed Hiroshima, “X-raying” Egypt’s pyramids in search of secret chambers—had earned him renown far beyond academia
NB: Walter Alvarez is the son of Luis Walter Alvarez the Nobel Physicist, that has done it all, including early work on the K-T Boundry with his son.
Also, note the minor(?) difference of 200k years between the two camps?
What caused Alverez to focus on the impact clay layer was a vast abundance of foraminifera in their trillions below the layer, and a near total absence above it.
That's why they did a iridium measurement in the first place, if the thin clay layer was deposited in a very short while there would be little micro-meteor deposited iridium incorporated in it.
The more iridium the more micrometeors would have had time to contribute their little bit to the total.
They found a HUGE amount. Why? was the layer billions of years in the making? Or was the iridium delivered in one big lump? The knew from other dating methods that the layer could be no more than a few hundreds of thousands of years.