Bismihi ta'ala
The observation of the 'iddah period is not only to ascertain pregnancy, or not, but rather for many issues, and most importantly it is a divine ruling placed by Almighty Allah.
As long as the marriage has been consummated, even though ejaculation did not occur, when the husband divorces his wife, she will still need to observe 'iddah.
There must be a "cooling off" period for the purpose of emotional detachment, recovering from previous relationship, transitioning back to another life, society realising their marital status, and more reasons that would support the importance of 'iddah for divorce, or 'iddah for passing away of husband.
And Allah knows best.