I think the use of the term "hepatitis" is confusing. There is infectious hepatitis, like A,B,C, etc. There are vaccinations for types A & B. Then there is inflammatory hepatitis, which is not infectious in nature, it only means the liver is inflamed. Since these articles are only using the generic term "hepatitis" and not "hepatitis A,B,or C" then we are talking about inflamed livers.
Adenoviruses do cause inflammation, and were found in about half the children, however most of the kids were not vaccinated for covid, so a link to a vaccine like J&J is unlikely.
Possibilities are: A new strain of adenovirus that inflames the liver. Viruses mutate all the time, this woudn't be much of a surprise. OR this could be a new version of Multisystem Inflammatory Syndrome in Children ( MIS-C) associated with a history of infection with a new adenovirus or Covid ( symptomatic or asymptomatic), only preferentially affecting the liver.
I've taken care of kids with post-covid MIS-C and that seemed to mostly affect the heart, but that was also earlier in the pandemic so other variants could act differently.