They would, I agree.
On the other hand, from the Ukrainian statements, it's not clear if the US actually shared any of the evidence with them before making their statement, which means the Ukrainians could well have thought it was a Russian strike. Remember, they may well have launched an interceptor at a missile heading in that direction to begin with.
And a 5V55 reaching Poland from Belarus is not a technical impossibility. It's beyond the range of the guidance system, but not beyond the range of the missile itself on a ballistic trajectory (long-range SAMs are fast, which translates to range on a ballistic trajectory). Likewise, while the missile is supposed to have been retired by Russia, the same is true of the T-62, especially older models like the obr. 1967
And on top of that, a stray missile can go a very long way in an unexpected direction. It's possible Ukraine hadn't fired any S-300s near the border at all. For context, a Syrian S-200 launched as an interceptor once came down in the Negev desert. If Ukraine hadn't fired anything remotely near the border, they may have been quite surprised by the idea of it being a Ukrainian missile.
I think the American/Polish position is most likely to be correct, but they absolutely have to involve the Ukrainian side properly, including sharing all available evidence. It looks like it is getting smoothed over now, which is good.