Trump was the boy.
Everybody knows how that story ended, but as a reminder:
"This tale concerns a shepherd boy who repeatedly tricks nearby villagers into thinking a wolf is attacking his flock. When a wolf actually does appear, the villagers do not believe the boy's cries for help, and the flock is destroyed. The moral of the story is that liars will not be rewarded; even if they tell the truth, no one believes them. "
There's a cost to lying. Sometimes it's your own flock. Sometime's its everybody's flock. Maybe Trump was right, maybe he wasn't. The boy was right about the wolf, eventually, too. The moral remains the same.
If I had to pick a "moral" of that fable, it's to never let your guard down, no matter what.
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.