Perhaps you should read the articles on the subject, see what they say. Two things I've noticed: First, the FBI investigated the claims, when they were originally made, and found they were false. Smirnov (the informant) claimed how he had conversations in 2015 and 2016 how Burisma hired Joe and Hunter to run interference for them. The issue is, there was no evidence that he spoke to them in 2015 or 16, that the first contact actually was in 2017. And this is beyond the investigation being unable to support the claims. It was also found that Smirnov was strongly opposed to Joe Biden's campaign in 2020 and that is likely a primary reason for his approaching the FBI with these claims.
Next, when they reinterviewed him in 2023 (knowing that he had lied about talking to Burisma executives in 2015 and 2016), Smirnov changed his story to having heard it from the Russians.
Now, we'll see what might occur with these charges but, based on the evidence released so far, it appears that there is good reason for Smirnov being arrested for lying to the FBI before he could leave the country.