Learn English – What does “he’s only telling it how he sees it” mean? [the question was rewritten]

he’s only telling it how he sees it.

Could you please tell me what's the meaning of the sentence above?

Is there any idiom here?

Note: The question has rewritten. So the(colleenv)'s comment refers to the original version of question.

The full text is:

Beverly listens to him, licks her lips nervously, and waits. No one else
volunteers anything.
Finally, she blurts out, “There’s something I should probably say.”
All eyes turn her way. She almost loses courage. She doesn’t know if the
argument between Dana and Matthew is relevant or not, but it will certainly
sound damning.
“What is it?” David says calmly, as she hesitates.
“I heard them arguing, last night.”
“Dana and Matthew?” David says, as if in surprise.
“Yes.”
“What was the argument about, do you know?”
She shakes her head. “I heard them shouting, but I couldn’t make out any
words. Their room is next to ours, on the same side of the hall.” She looks at
her husband. “Henry slept through it all.”
“What time was this?”
“I don’t know, but late.”
“Did it sound . . . violent?” David asks.
“I don’t know. It was just raised voices. No crying or anything. Nothing
slamming, if that’s what you mean.”
There, she’s said it. If Matthew’s done something wrong, then it’s good that
she’s told them.
David can sense the heightened distress of the others. They don’t like
what Beverly has said; it makes them uneasy. They don’t like to think
the unthinkable. He can see from their faces that they are all
imagining it—the argument, the push down the stairs. He’s sorry for
their distress, but he’s only telling it how he sees it. It doesn’t
seem possible that Dana could have been injured like that from her
fall, and he doesn’t want them messing about with the body. And now
this new information—Matthew had told him that he and Dana had not
argued. If Beverly is to be believed, Matthew lied to him.