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the article has a fairly neutral tone so I'd say it's most likely true

Not a great way to evaluate veracity. Similar articles from the same outlet will casually refer to AES states as dictatorships, for instance. The nonchalant tone just signals "everyone knows this and you're a fool if you dare question it."

This article pulls from research by a Cuban-born, Cuban-educated academic (El cuerpo nunca olvida by Abel Sierra Madero). That research would probably be the best take on the subject if someone better at Spanish wants to track it down (it mentions interviews with actual participants). From the El Pais article, even reading with a critical eye, it seems fair to say:

• From 1965 to 1968 the Military Units to Aid Production (UMAP) system existed, which served as "served as centers of production and political correction." I'd call those "labor camps," although the article provides little information on typical conditions (though it does hint at some bad anecdotes).
• There was at least "the pretext of performing compulsory military service," and "homosexuals and religious people predominated." Not a lot is said about who had what intentions with respect to the program, but it's clear there was some prejudice.
• 30,000 people passed through UMAP out of a 1965 population of 8 million, or about 0.375% of Cubans.