I don’t think this constitutes such an admission.
From the article: "Uvarov and his friends from Kansk in Krasnoyarsk region, Siberia, were also accused of experimenting with real explosives and were training "for the purpose of further terrorist activites”.
Lawyers for the teenagers said they were playing in empty fields and had no intention of causing harm."
The juxtaposition of the sentences means it could be read that way, but it is far from a clear admission. He does not say “playing with explosives.” And are we talking about fireworks here or what?