Here's the explosion video again if you don't like telegram link.
Pro-Ukrainian sources are claiming this is "38 old ballistic missiles (SS-24) with 1,800 tons of rocket fuel. They were stored at the Pavlohrad Chemical Plant. They were not decommissioned due to lack of funds" instead of S-300 missile.
I would think both. The big massive explosion is probably the chemical plant, followed with subsequent strikes and cook offs of the S-300s and ammo trains nearby. Smart or stupid of them to store weapons and ammo near the plant Pavolograd is an important transportation hub for east and south Donetsk, it's obvious it will be protected and surrounded by your best AD, which unfortunately has gone up in smoke. There are numerous videos showing windows blasting which is apparently a massive shockwave in the area. More than depots there are ammunition and supply trains in the area that would have been targeted. I would think the culprit are Iskanders. Bombers on the air like Tu-95s are for followup strikes all over the front.