In teaching, what do we actually mean when we say "syllabus completed"?
This sounds a lot like a "rantion". :) I'll try an answer, but most of what I writing is a truism.


  Does it make sense to just say that syllabus is completed in which students get almost nothing?


There are two different questions: was everything that was on the syllabus covered in the class? and were the lectures helpful and sufficient to learn the required material?. Both should be asked to the students. They should not be mixed up.


  Is it not worth teaching some small amount but in an optimal way so that the students would get the true knowledge from the course and could apply whatever they learned?


Clearly, there is a balance to be found between amount of material covered and depth/effectiveness of the lectures. For each course there are different reasonable balance points, and it's the job of a competent teacher to find one of them.


  What do we actually mean by the term "syllabus completed" -- Do we mean "students learned everything that we taught" or "we taught everything, but don't care about students learning"?


The second. The first one is a separate question, equally important (see above).