Here in New York City, for example, it's abundantly clear that poetry is thriving. Not only are there huge, prominent sections for poetry books in our bookstores, along with sections for poetry anthologies and poetry journals, but we also have at least one bookstore which only sells poetry, and one library, as well. There are poetry readings that happen every day, and at least two bars and clubs devoted exclusively to hosting them, with several others that primarily host them. The poetry book fairs that happen once or twice a year showcase hundreds of newly published books. Lots of other places are like this. I can think of poetry-only bookstores in Boston and Seattle, for instance. And City Lights bookstore in San Francisco has a whole floor dedicated to poetry.
Of course, poetry books are not going to sell as well as J.K. Rowling novels. But in terms of the "health of the medium" and the "impact on culture," it's never been better. If you don't see that, it's likely because you live in a place that's at a remove, culturally or geographically, from a cultural center like New York, or from a place with research universities. So who is "insular" and "out of touch," exactly?
He goes on to quote some numbers that make it seem like poetry books are secretly huge sellers, but implies there is a sort of conspiracy keeping us from acknowledging it. I don't find that likely.
Bookstores are extremely profit motivated, and not averse to changing their priorities to sell what people want to buy. Science fiction, fantasy, and other genres used to be ghettoized in the back of the store: now they are well-stocked and prominent, because they sell comparatively well. Look at the proportion of shelf space given to poetry books, and where they are located in the foot traffic patterns of bookstores, and I'd suggest that's a good index to its real sales volume, new and used copies included.
There are unusual situations where poetry books sell well: I remember when the singer Jewel published one. That moved some copies. But that's not typical of the performance of the rest of the thousand or so volumes published every year, nor of the health of the medium as a whole in terms of economics or impact on culture.
I appreciate that the author is making a spirited defense of something he loves, but I think it's incorrect, and it seems insular and out of touch. It seems to be denying reality. The idea that poetry is actually doing great implies that it doesn't need to change in order to be relevant to its audience, that the only thing missing is that we need to wake up and realize everything is totally fine.