Originally native to North America, where it grows wild, so if its growing wild here, it'll be a garden escape, as you put it.

According to Wikipedia it's native to forests in western North America, which means it was imported to the UK, presumably for gardens. If you find it in the wild I guess it's a garden escape. I don't think it's found much outside gardens.

Snap Bamboo, we posted together! :-) Good job we gave the same answer!

and I agree with you both. it is not a native so it is a garden escapee.