https://www.worldometers.info/coronavirus/country/us/ right now shows 12850 cases per 1M in NY, which is 1.2%. Which is pretty close to what the study shows for LA. So it's not actually suggesting NY has much more than it's already known, if we assumed overall percentage would be similar for both. It does suggest California - which currently lists 853/mln - undercounts it by more than 10x. Of course, provided that the study is true.
Also, having antibodies and being hospitalized is a very different thing. If a lot of the cases are asymptomatic or very light you won't see hospitalizations and you'd never know the person had an encounter with the particular virus until you do mass testing. And NY is quite different from LA, as far as I know, in terms of how dense population can be and how much people use mass-transportation and other high-risk scenarios, and the probability of being hospitalized may also depend on that (in other words, it may matter not only whether you had any contact with the virus, but in which circumstances and how much of the contact you had).
That doesn't prove the study is true of course, but I don't see anything from comparison with NY that makes it implausible on its face.
Also, having antibodies and being hospitalized is a very different thing. If a lot of the cases are asymptomatic or very light you won't see hospitalizations and you'd never know the person had an encounter with the particular virus until you do mass testing. And NY is quite different from LA, as far as I know, in terms of how dense population can be and how much people use mass-transportation and other high-risk scenarios, and the probability of being hospitalized may also depend on that (in other words, it may matter not only whether you had any contact with the virus, but in which circumstances and how much of the contact you had).
That doesn't prove the study is true of course, but I don't see anything from comparison with NY that makes it implausible on its face.