I doubt robbers would build a tunnel big enough for several dump trucks. it sounds more like something official.
Then it probably would be an allocation for the MRT/ Metro Rail Transit. If you have rode the the MRT train here, its not like PNR/ Philippine National Railway which is on the ground, or LRT1 and LRT2/ Light Rail Transit which is above ground. The MRT was built not to impede the traffic in EDSA, it could go above ground, underground or ground level. So probably, it was a scrapped allocation for the MRT.
Is it plausible to be several dump truck wide? Yes it is. Is it plausible to be an underground tunnel, i hardly doubt it but still plausible, why? By studying tunnels like done in Iwo Jima, Singapore, and Bataan&Corregidor in the Philippines, Most of these tunnels are not several dump trucks wide, most tunnel are passable by only at least 2 men, and a lot are even only 1 man passable.
But of course, everything is possible and its still a great discovery. My question is, why build a tunnel as wide as several dump trucks? Tunnels back then are made for secret passage to an area, but several dump truck wide would be incredible purpose.