A "stray missile" was never a tenable explanation. Unless my situational understanding is goofed up, Russian missiles would have come at Lviv from either the Black Sea or the Donbass. That would be in the southeast quadrant from Lviv. If you have a point defense system, you would detect threats coming from the southeast and would fire your interceptors in a southeasterly direction for a nearly head-on engagement geometry. Any missile that was fired and ended up in Poland would have been fired along a northwest trajectory, completely opposite to where it would have been fired to perform an intercept. So, the conclusion of an intentional firing is supported by circumstances.

It turns out the wreckage was photographed at the scene, with photos on the internet. The wreckage was quickly identified as being part of the propulsion system for a S-300 interceptor, according to part and serial number. In the face of photographic evidence of identity, there is no wiggle room to claim anything else. Even in the face of this, Biden still wanted to wiggle.

And, even at face value, a missile targeting a tractor and killing two bystanders is not going to cause World War III. The Poles would have to be as deranged as Zelensky to make this a casus belli. But they are already pissed-off at him because he shopped this flagrant lie right out in the open, totally blowing his credibility.

I expect Trump got briefed with the news after the speech, and he probably found it greatly humorous.