Sure, you can make the case for always ignoring past lies, and always evaluating every claim based on current evidence. The reason the story exists is to try to illustrate how most humans actually behave, despite there being a preferable response.
In addition, the evidence for the lab leak theory wasn't strong back when Trump became the mouthpiece for it. There wasn't much of a reason, even if you evaluated the current evidence for what could be another one of his thousands of documented lies, to take it particularly seriously.
That situation might be changing now, and we are seeing that in the media and culture right now, as we respond to new evidence, or more specifically, lack of other expected evidence.