Lopez is/was a confirmed stoner; I've seen numerous pictures even just on this forum of him and a fan after blazing, after emerging from woods having blazed, etc. Similar stories of Mendez come as no surprise; I believe it's inferable from some chapter of the Opeth biography that the Martins toked, as Mike mentions their then-low tolerance of hard alcohol and one of them vomiting in a potted plant or something.
Weed is a psychedelic depressant-stimulant-intoxicant; that's a pretty broad classification. It can calm you if you're already somewhat calm or trying to become calm, and it can prompt anxiety if you're already somewhat anxious or under stressful circumstances. The word 'psychedelic' means "mind manifesting"; psychedelics simply hold up an augmenting mirror to how you already feel. Hence, weed can both depress and stimulate.