But we also already know that we don’t achieve sterilizing immunity from coronavirus even with previous infection or vaccine. So it was never the case that we couldn’t stop transmitting the disease.
"people who were exposed to H1N1 flu last year will become immune to it, so once it runs its course people will get herd immunity from it and the virus will die out".
that's not how it works. What the scientist is saying here is that it's mutating fast enough that by the time we start to approach "herd immunity" (via vaccination or just exposure) it's mutated and the herd immunity is no longer effective against the new variant. So yeah you're immune to the strain from last year, but it doesn't do anything against this year's strain.
and on top of that Delta is literally so infectious that it'll just about spread right through the existing vaccines. The vaccines were 98% effective against the initial strain, now Pfizer/Moderna are 88% effective, which means you have 6x as much spread (at an epi/population level), on top of a strain that is, itself, 2.5x as transmissible. So now we have to hit a much higher level of vaccination (near-100%) to get the reproduction number under 1.0.
And then you've got the other vaccines - some early studies indicated Astra-Zeneca is only about 60% effective (!) which means even with 100% vaccination that vaccine could not stop the spread. Best-case scenario you have to get those people a booster shot of something else - and the clock is still ticking, it is still mutating and in another year the effectiveness of all the existing vaccines will be even lower.
And on top of that, those are only the "severe illness" numbers, the "mild/moderate" illness numbers are higher, vaccinated people still spread an appreciable amount, and it's spreading through kids very aggressively now (funny how HN told me just last week that kids were immune and we could reopen schools...)
In short if you want vaccines to actually work and reach herd immunity - you have to slow it down enough with containment measures to slow down the rate of mutation. Because if you just let people get exposed, every exposed person is another chance for the virus to mutate, and it's mutating fast enough that by the time we get everyone vaccinated, the virus will have mutated enough the vaccines are no longer very effective.
(And reality check - we've barely got a start on vaccinating the global population, which is what you really have to do to control it. Vaccines are flowing like water in the US and Europe, and to a lesser extent in other developed nations, but in a lot of the world you just can't get a fucking vaccine even if you want one. A few months ago the number was something like 140 countries hadn't even gotten a single dose of vaccine yet.)
Realistically that is no longer possible at this point. People are done with quarantine and social distancing and 40% of the country won't get vaccinated even when they're free and readily available. And now it's mutated to become several times as transmissible - that's game over. It's over, it's going to become endemic, it's a foregone conclusion at this point.
I suggest you read up on "sterilizing immunity". People with immunity against COVID still can transmit the virus. This was already known so it's nothing new. There are some viruses that we have sterilizing immunity against, like smallpox. There are other diseases like tuberculosis, COVID, etc where immune people still shed the virus.
And you can't stop the rate of mutation because animals can get and transmit SARS-CoV-2. So it doesn't matter if all humans were immune, animals could still get infected and cause mutations that re-enter the human population.
It looks like that also is affected by age and other constraints: https://www.nature.com/articles/d41586-020-00502-w