I also agree that logically one can declare ownerless something that is not in one’s possession. But how does that fit with this Talmudic passage:
Babylonian Talmud, tractate Bava Kamma 69a
“And the modest ones would place down the money and say: Whatever is picked from this shall be redeemed onto these coins. And if you should say: Who is the tanna of ‘the modest ones’? Rabban Shimon ben Gamliel — but Rabbi Yohanan would not rule in accordance with an unattributed mishnah that reflects the view of a lone individual! But didn’t Rabbah bar bar Hannah say that Rabbi Yohanan said: Wherever Rabban Shimon ben Gamliel taught in our Mishnah, the halakhah follows him, except for the cases of guarantor, Sidon, and the last proof? They said: Do not say, ‘Whatever is picked from this,’ but rather say, ‘Whatever will be picked from this.’ And did Rabbi Yohanan really say that? But didn’t Rabbi Yohanan say: The modest ones and Rabbi Dosa said the same thing — and Rabbi Dosa says, ‘was picked’! As it is taught in a baraita: Rabbi Yehuda says: In the morning the owner of the house stands and says, ‘Whatever the poor will pick today shall be ownerless.’ Rabbi Dosa says: Toward evening he says, ‘Whatever the poor picked shall be ownerless.’

In other words, what is the one thing that the modest ones and Rabbi Dosa said? Seemingly, we have to explain that they said it is possible to declare ownerless / redeem / consecrate something that is not in your possession, no?

As a matter of Jewish law, it is clear and agreed that one cannot consecrate something that is in the hands of a thief (according to one view because it is not his, and according to another because it is not in his possession), and in the passage here they seemingly compare ownerlessness and redemption to consecration.
But that is exactly the discussion there, and the above-mentioned Ketzot HaChoshen addressed it. The question is whether one can effect redemption or ownerlessness of produce that is in another person’s possession or not. Alternatively, both cases may be speaking about the law of retroactive clarification, and not about an act performed on produce that is with someone else. Those are the two possibilities there in the Talmud. The question is what remains as the practical halakhah, and that is discussed in the sources I cited above.
By the way, when the produce is in the hands of the poor, it seems logical that one should distinguish that from our case, since the poor themselves assume that the produce is ownerless and are interested in that ownerlessness. So here there would be room to say that the homeowner can do this as well. After all, those to whom it belongs and those in whose possession it is are both in agreement.