The key word from Trump Jr., according to University of California Irvine election law expert Rick Hasen, is “solicit,” which has a very specific meaning in this context. To quote the relevant statute:
A solicitation is an oral or written communication that, construed as reasonably understood in the context in which it is made, contains a clear message asking, requesting, or recommending that another person make a contribution, donation, transfer of funds, or otherwise provide anything of value.
Trump Jr. was clearly soliciting information that he knew was coming from a foreign source. Given that political campaigns regularly pay thousands of dollars to opposition researchers to dig up dirt, it seems like damaging information on Clinton would constitute something “of value” to the Trump campaign.
The solicitation bit is why it doesn’t matter if Trump Jr. actually got useful information. The part that’s illegal, according to the experts I spoke to, is trying to acquire dirt on Clinton from a foreign source, not successfully acquiring it. And his statement more or less admits that he did, in fact, solicit this information.