Why not? I understand that the questioner recited the blessing himself and did not fulfill it by hearing someone else.

Because that isn’t a blessing. Moving your lips is not speech.

See Halikhot Shlomo, Prayer vol., p. 334-335, where he brings from the Talmud and the Shulchan Arukh, regarding the law of someone who was standing at “and you shall write them,” that if he began “so that your days may be multiplied,” it shows that he continued by force of habit, and it is proven there that he has fulfilled his obligation. See there.

Indeed, that does seem to follow from the words of the Shulchan Arukh, Orach Chayim 64:4, and still the matter is difficult, as Rabbi Shlomo Zalman himself brings from Rabbi Chaim.
And in Havdalah, where one has to intend to distinguish between sacred and ordinary (it is not merely a text that needs to be said, as in the Shema, where perhaps there is room for that reasoning), it seems even worse.

With God’s help, it is written in Adar I, ch. 2:

On the face of it, someone who is holding a cup of wine and lighting a candle, etc. is aware that he is performing Havdalah. There is full intention at the beginning of the act, and a “dim intention” afterward as well (as the Chazon Ish said regarding prayer too).

The intention of “standing and speaking before the King,” which according to Maimonides is indispensable based on Rabbi Chaim’s explanation, is a rule in the laws of prayer, where a person is “speaking with the King,” and it was not said regarding blessings. And even in prayer, although according to Rabbi Chaim the intention of speaking with the King is required for the entire prayer, the halakhic decisors took the view that it is required only for the first blessing of the Amidah.

Regards,
Chasdai Betzalel Duvdevani Kirshen-Kvas

And the joke tells of a fellow who went to India “to find himself,” and found himself at Yishtabach…