The use of the glottal stop for the /t/ in "Manhattan" is how New York City natives born throughout the 20th century generally pronounced it, other than possibly elite class speakers. And you're correct that such glottalization occurs in other contexts in many regional US accents. The suggestion that this might be a result of recent British English influence is absurd.

It's actually more likely to be a holdover from very old British English influence - the Irish and Scots press-ganged white slaves who fled into the hills of Virginia and became the rednecks of coal country. I know that's where I get most of my dialect, even though I grew up in Indiana (so I also get a Blues-Brothersish Chicagoan avoidance of theta, to end up with that unique Hoosier accent that non-obsessives don't notice).

London doesn't have a monopoly on glottal stops.

There are a lot of incorrect details in the article, although the premise is mostly accurate; the use of "spot on", for example, is certainly increasing on this side of the Atlantic, along with other phrases that aren't mentioned, such as "bog standard".

But a lot of the article's examples aren't quite right: "to book" a room or table has always been in use here; it just sounds more colloquial than "reserve", so the latter is what would be more likely printed in hotel brochures and restaurant menus. Conversely, "expiration date" is used in formal or ersatz-formal language, but "sell by" is what's actually usually printed on supermarket labels.

Some of the items ignore subtle distinctions in the way certain phrases are used. For example, there's always been "washing up" in America, but it's invariably done before dinner, not afterwards.

Source? Labov's Atlas of North American Atlas says that changes among dialects in the US are more pronounced now than they were 50 years ago. If we're not even converging among ourselves, how can we all be possibly converging in any meaningful way with people overseas? It's simply not documented in any systematic way.

I grew up in Minnesota and moved to southern California as an adult, and after listening very hard for any trace of an accent out here, the only thing I've noticed is that Caucasians who grew up here and went to AP english classes enunciate their T's very clearly. It's kind of weird because that kind of kid in the upper Midwest is very careful to turn the T into a glottal stop, to fit in with everyone else.