the first '-na-' shows the action has been/is being done in the present tense, so it is already "said". Your translation implies that making the statement is possible, but doesn't indicate that people already say it. Yours would be 'Inaweza semekana kuwa/kwamba vifaru ni wachache', '-weza' implying 'can be' instead of 'is'
I see what you are saying, but shouldn't that logic also apply to, for example, mlima unaonekana, which is translated to "the mountain is visible" / "the mountain can be seen"? If we follow your explanation, the best translation here would be "the mountain is seen". Is this correct?