What is true is that disorganised people often won't succeed at the top unis, so it's in noone's interests for them to get in.
I don't like the term lazy: it's too perjorative. Being organised and hard-working requires a set of skills that not all students have succeeded in acquiring at a given stage; that's a better way to look at it.
The education offered at the top universities is typically predicated on students having those skills already; if they haven't, they'll really struggle.
That said, any learning environment worth its salt will attempt to help students acquire whatever skills they need. I often see students struggle with organisation early on, sort it out, and go on to succeed. Sadly, my department doesn't have the resources to hold students by the hand and help them develop these skills, and I also see students fail to sort it out and drop out/fail. It's easier to sort out if both:
- the course is not already very hard for you intellectually (if you can only keep up by dint of consistent hard work, you have no leeway if you're not working like that from the start): for this reason, students who lack those skills may be better off somewhere with a relatively less challenging course;
- you get quick, personalised help to spot that you have a problem and help you sort it out. This is why small departments can be good.
A college system, as at Oxford or Cambridge, can help in the second way, but not the first! They are almost the only places in the UK that have enough selectivity and enough effort to devote to admissions that it's close to true that you don't get in with major skills deficits - but it's all relative.
It's never too late to acquire these skills! However, there's certainly a risk in going somewhere that will assume you have them and not be hugely good at helping you acquire them.