It wasn't 100% clear in the article.
They state they had"zero warning" but the word "denial" saiz to me the opposite if asked beforehand. i.e.
USA "It looks like you are about to test a nuclear weapon, Are you going to conduct a nuclear test"
India "No we are not"
in which case the USA did have an idea india was about to conduct a test hence asking and receiving a denial. This is similar to USA saying russia was going to invade ukraine and russia denied it.
Though perhaps they are using the word afterwards as in
India "We have not had a nuclear test, your manmade earthquake measurements are wrong, such a explosion never took place"
In short I'm not sure what the word denial is referring to
EDIT: OK looked a bit more. It seems USA was unaware beforehand and india did admit to conducting the tests afterward, thus I think the 'denial' is just the article writer miswriting. It seems part of the reason USA was unaware is due to india being aware of when the US spy satelittes passed overhead and also doing the preperations at night.
2 points that make it easier for USA to detect now, 1. tech is 24 years improved, 2. USA are aware something could go on thus will be watching like hawks unlike the 1998 india test which came out of the blue.
Then again how easy would preparations be to detect. Surely the whole point of nuclear weapons is for them able to be shot off within short notice, I'm pretty sure Russia or USA could launch a nuclear missile and the other side not being aware until the thing has left the ground/sub.
EDIT2: To make it clear, sorry I'm a terrible explainer.
But to me the news about US etc hasn't seen any indication that Russia is about to launch a nuke, to me sounds just like a statement made to reassure the population, if russia/usa wanted to they could launch one in 5 minutes time, thats the whole point of MAD