Wow, looking at the history of the ARM generation the original versions of the Raspberry Pi uses, it’s hard to believe it’s so old! When the Raspberry Pi B+ was released (2014), the ARM core it used was already 11 years old (using the ARM1176 core from 2003). So it’s not unbelievable that you might need to start supplying an arch flag to produce compatible code building on a different platform (like the newer Raspberry Pi the article says they first built on).
As others have said, it does seem like a misconfiguration (perhaps in the defaults shipped by their distribution) that the correct arch is not picked by default when building on the Raspberry Pi B+ itself.
As others have said, it does seem like a misconfiguration (perhaps in the defaults shipped by their distribution) that the correct arch is not picked by default when building on the Raspberry Pi B+ itself.