I found that API testing is supported by UFT. What about LeanFT. I know I could do the testing with help of some Java libraries, but I am not sure if test results can be updated in HP ALM. What is the trick here? Can I or cannot do the API testing with LeanFT?
LeanFT is relying on an SDK (Java, C#, JavaScript, etc.)
Any of those SDK already have plenty of ways to do REST API tests, so LeanFT didn't implement a way to do that. (It would mean to reinvent the wheel).
Your question really is: can you report results to ALM through LeanFT? And you can do that, of course. Here's one example, out of many others.