Non selective

yes but probably not as much as a selective sixth form. ps: grammar schools are selective there's no thing like non-selective grammar schools. if u were there since year 7 you can go straight into sixth form but if you are moving you need a rquired amount of gcse grades from the schools, check the school website

I’m applying sixth form to an academically outstanding school in Northampton, which is called Northampton school for boys, I did in fact email them, they are a grammar school, but they said they are not selective, I’m confused.

I don't know this particular case but it could just be that they used to be one and are now no longer selective. They could also be partially selective or they're using the now rather antiquated meaning of 'grammar school' in that it's a school that teaches Latin.
But yes, you can get into Cambridge - and by extension other Russell groups - from a non-selective sixth form college, titularly 'grammar' or not.

I’m applying sixth form to an academically outstanding school in Northampton, which is called Northampton school for boys, I did in fact email them, they are a grammar school, but they said they are not selective, I’m confused.

I understand now. Grammar schools are selective but if you are an external applicant going into Sixth Form its your GCSE results that matter not any other tests. Some may say you need a set amount of points (eg 9 in GCSE is 9 points) such as 44 or up to about 64 to do subjects like Further Maths. If you joined in any other year, yes it is selective