Geologist here: I've studied in 1997-2003, than I've had a PhD in stratigraphy in 2007. I researched on a similar subject trying to identify what caused "dinosaurs" appearence (spoiler: another extinction event). At that time the two hypotheses, at least in Europe, were still presented as equally valid.

How these two phenomena can be equally the cause of the mass extinction at the end of Cretaceous?

Two reasons, at least. The first is the uncertainty about how rapid was the decline of Life. Any geologic date has an error, so it is extremely hard to assess the velocity of geological events. Around the K/T boundary most rock samples cannot be dated with a precision greater than a million year, that means that 2 separate events have the same probability to be coeval or separated by hundreds of thousands years with 95% confidence. In these case the uncertainty cannot allow us to distinguish an abrupt event from a slow one. More precisely, some rocks can actually be dated with an error of ~10000 years, like the iridium anomaly, or like the Deccan's lava but forams does not live inside lava. So you have to correlate events, to find proxy signals like changes in water geochemistry and so on. This increases uncertainties when dealing with fossils, and may be an easy way to denigrate palaeontological studies, but there are still evidences of a progressive decline.

The second reason is that the decline in marine ecosystems was sharp after the iridium anomaly, but we cannot explain why that precise impact killed big reptiles on sea and land whereas hundreds of other asteroids that hit the Earth during mesozoic had not. It was surely not the asteroid impact alone. There are evidences that all the mass extinction events are not instantaneous, but they are all related to a sequence of (misfortunated) events in rapid sequence. The emplacement of igneous provinces has proven to be a common menace for life on Earth. Asteroids too.

What we actually "read" in the rocks is that Deccan's traps paved the way, the Yucatan impact provided a massive stroke, and the closeness of these two events finished the survivors. In my opinion the "war" between scientists was more an US phenomenon related to their need to get access to NSF funds. The cool one got all.

The article is, roughly, about a geologist who studies exactly forams and disagrees.

Yes, and others who study forams have looked at the same evidence and come to different conclusions. Someone's inferences are not following the same logic as others, and the most parsimonious explanation is that it's this person -- who I understand has become somewhat notorious -- not the rest of the scientific community.

And you know better because...?

The main point of the article is that the issue is far from settled, and that evidence for both sides is lacking. Until the consensus is reached through scientific means (instead of bullying) the matter is not closed.

Could you link to some of this purported notoriety?

The article states that some scientists disagree and have evidence to back their claim that life was on it's way out.