There are two aspects to your question.
The first is to try to determine if the text for Rashi’s comments to Tehillim 75:11 as found in the Sefaria version is correct and accurate or not.
Then, the second aspect is to address your primary question as stated in the heading and recapped in your closing paragraph.
On the first aspect, the accuracy of text of Rashi on Tehillim, the authoritative and critical edition of Rashi’s commentary on Psalms by Rabbi, Professor Mayer I. Gruber republished in paperback by JPS in 2004 is important.
As Gruber discusses in detail in pages 158-164, after reviewing over 60 extant handwritten manuscripts, most dating from the 13th, 14th and 15th centuries, he determined that virtually all printed editions, including Parshandatha by Rabbi Yitzchok Maarsen are corrupted and inaccurate. Gruber goes on to explain that he relied primarily on two handwritten manuscripts known as Oppenheim 34 and Vienna 220. The details of why are quite interesting and worth the read if you can find a copy of Gruber’s book, which is currently in print.
The correct text of Rashi to Tehillim 75:11 for the words, “וכל קרני רשעי׳״ is actually:
של עשו הרשע
There is no mention of Amalek whatsoever.
It then adds a final Mem to עגדעם to indicate that they, the Rasha’im from Esav will be cut down, and continues with the citation from Ezekial 25:14.
כעניין שנ׳ ונתתי נקמתי באדום ביד עמי ישר׳ ישר׳ יגדעו קרן עשו
So that resolves your question about the discrepancy concerning the mention of Amalek, which is not found in the Navi Rashi cites. The correct text of Rashi is consistent and accurate with the appropriate Navi.
The second aspect of your question, about what the meaning of this horn is in regard to Esav, can be found in the introduction of Rabbi Eliezer Fishel ben Rabbi Yitzchok of Krakow to his super commentary Parshat Eliezer to Sefer Karnayim with the commentary of Rabbi Shimshon of Ostropole.
In paragraphs 3 and 4, he explains how the concept of horn (קרן) as it pertains to Israel in the context of the final redemption means Moshiach like is found in Habakkuk 3:4 which says, “קרנים מידו לו”. He goes on to explain how this is referring to both Moshiach ben Yosef and Moshiach ben David.
As he explains further on in the book in the 14th discourse, just as there is a Moshiach for Israel (actually two, ben Yosef and ben David) the side of Holiness, there is also an anti-Moshiach for the other side. In the context of Tehillim 75:11, (וכל קרני רשעים של עשו) the Moshiachs of the Rasha’im of Esav.
And this principle, including Rashi referencing Israel and Esav is in keeping with Bereshit 25:23-24 which addresses the conception and birth of the fraternal twins, Yaacov (Yisroel) and Esav. Please take note of Rashi’s comments to the citation from Bereshit.
