I am told by some people that the companies that follow the Agile methodologies can achieve only up to CMMI ML 3 and they cannot aim and achieve CMMI ML 4 or 5.
Is this true ?
As my mother in law would say: "I'd agree with you if you were right." There is much confusion and mis-information about Agile in the industry. Everyone talks about it, but few really understand and can execute on the methods. Everyone thinks it means "no documentation" and that is just wrong. It does mean we should make intelligent decisions about the amount and type of documents that are produced, and that we should remain vigilant about focusing on what's important (the software and the customer) but that is a different issue.
Conversely, there are too many people that think the CMMI means "lots of documents" and that ML4/5 need even more. This too is untrue. The CMMI tells us that if we perform a process there will be evidence of it being performed. That's different than saying "if we create documents we are performing a process."
I am working with one such Agile organization now that is about to complete a ML4 appraisal - and the are finding ML4 much MORE valuable to them than they did ML2 and ML3.
One reason for this is that at ML4 we have good data to tell us how we can minimize and optimize our process. Agile proponents always talk about "just enough" (and I think that's great) but until you reach ML4 and have collected the right data to perform OPP (statistical analysis) how do you KNOW what "just enough" is? The answer is "you don't."
This organization now knows which process assets bring them value, which ones don't, and where they should insert (or remove) a process component to further optimize their development process.
So, there is no truth to what you've heard . . . plenty of "agile" organizations adopt ML4 and ML5. The question I would ask first is, are the companies you're hearing about really "agile" or are they that other, new-fangled methodology: "lazy?"