According to the article, he disappeared in 1976 (48 years ago). Apologies for the pedantry.
they mean at time of being declared legally dead (1986) it's been 10 years since he disappeared (1976)
I'm pretty sure that the GP understood that, and was just being nitpicky about grammar. It's incorrect grammar to say "it has been 10 years" in this case.
At the time he was declared legally dead it had been 10 years.