Interesting point.
... Well. Since we're discussing the outcome of a 'no deal' Brexit, in that scenario, nothing would prevent her doing exactly that. She DOES have four months to make arrangements ... and significantly longer if in fact the chemicals needed could be stockpiled (.. or ARE being, even now).
It's not as though a 'no deal' Brexit kills off trade with the EU in any case. We'd still trade with the EU, still buy what we needed, just on terms not quite as 'cozy' as they were when we were a member of their trading club. What we'd be doing, at minimum, was arranging a stopgap between our need for them, and the end of a temporary delay of supply at the EU end of things.
Of course, we might just decide that the EU had become too unreliable to trade with ... in which case, it's the EU's loss, and the US's gain.
'Project Fear' will, of course, reflect none of this.