Yes. There's nothing special about a mobile phone number when it comes to SMS delivery. The underlying infrastructure company given in the article, Bandwidth, provides phone number provisioning and bulk service for Google's Voice product. On-net (one number hosted by Bandwidth to another number hosted by Bandwidth) might be slightly more of a hurdle to intercept or redirect but off-net is fairly trivial.

Heck, even with "port lock" enabled on a Google Voice number, that is the barest of security against an attacker who has any kind of access better than "retail store employee." Working for a telco with access to our back-end port system, access several other people had, I could forcibly acquire a number by simply checking a box that said I had verified a written LOA even if the losing carrier responded with code 6P ("port-out protection enabled").

So, yes, you're likely sitting in a security-by-obscurity, or at least security-by-slightly-more-difficult-than-someone-else, situation.