So,jcenter is a Java repository in Bintray (https://bintray.com/bintray/jcenter), which is the largest repo in the world for Java and Android OSS libraries, packages and components. All the content is served over a CDN, with a secure https connection.
JCenter is the default repository in Goovy Grape (http://groovy.codehaus.org/Grape), built-in in Gradle (the jcenter() repository) and very easy to configure in every other build tool (maybe except Maven) and will become even easer very soon.
Bintray has a different approach to package identification than the legacy Maven Central. We don't rely on self-issued key-pairs (which can be generated to represent anyone, actually and never verified in Maven Central). Instead, similar to GitHub, Bintray gives a strong personal identity to any contributed library.
If you really need to get your package to Maven Central (for supporting legacy tools) you can do it from Bintray as well, in a click of a button or even automatically.
Hope that helps!
I am also not sure how you figured out those are fake downloads. For sure the script that DDOSes Bintray from China won't use Groovy, but it's a still a valid download. Not for showcasing how popular Groovy is (they factor out those things when talking about the numbers), but for the raw statistics - for sure. The file was downloaded, wasn't it?