Are we certain Delta was/is more severe? Since it’s more contagious, might it also be producing a lot more mild/asymptomatic cases that don’t get reported? Severity seems like a difficult thing to measure reliably if you can’t be sure about the denominator.
Multiple countries have sustained mass testing of at least certain groups of population. The argument about undetected asymptomatic cases would be valid in early 2020 when tests were scarce, but now there are good continuous metrics from people who get tested even if asymptomatic.
Oh, I'm not following the news on Delta or any other variants much, however, I just know people who are measuring the prevalence of variants in the general population in reasonable ways which would definitely cover also asymptomatic people (one is mass testing - e.g. right now 100% of local kids are getting weekly tests in schools, and all hospital admissions get tested even if it's e.g. a car crash, but there's also the viral analysis in sewer system, which is a cool way to get a total perspective on large populations), so studies about the severity of Delta (whatever they are saying) should not be distorted by the particular problem of asymptomatic cases not getting reported, the researchers now have good tools to get the "denominator" part correct.