They haven't just "found single neurons which trigger memory". They found the only neurons that were active while experiencing a specific situation and memorizi

They haven't just "found single neurons which trigger memory". They found the only neurons that were active while experiencing a specific situation and memorizing it and they managed to show that activating only those neurons is enough to trigger a reminscence of the very same experience. You cannot say some other neurons would trigger the same memory, because nothing changed about those neurons during "generating" this memory (under the assumptions science currently holds about the brain), so they cannot store it.