Here's the way I understand it. In a nutshell: He says that "נקדש... כשם שמקדישים means that we are just quoting the Malachim, but we ourselves are not saying their Kedusha. נקדישך...כסוד שרפי קודש, on the other hand, means that we ourselves are saying the Kedusha of the Malachim. During the week, our kedusha is limited to the one sentence of ימלוך. On Shabbos, we incorporate the Kedusha of the Malachim into our Kedusha by using the words they use.
Now to explain it as well as I can: I'll start with the essential Marei Mekomos.
Tosefta in Brachos 1:11
Sifrei Haazinu on Passuk 32:3.
The Tosefta says that Rav Yehuda said Kadosh and Baruch with the Shli'ach Tzibur. The Rogatchover learned that to mean that the Rabanan argue with Rav Yehuda; the Rabbanan didn't say either Kadosh or Baruch; all they said was Yimloch, which is the kedusha that is specific to Klal Yisrael. Rav Yehuda, on the other hand, holds that the tzibbur does say the entire kedusha. The basis of the machlokes is this: when you quote someone else's words, does it have the legal effect of your having said the words yourself. For example, if you were a witness to someone's blasphemy, and you testify against him in Beis Din, and you repeat his words verbatim, are you guilty of blasphemy? This, he says, is a machlokes between our girsa in the Gemara in Sanhedrin 56 and 60, which says that nobody is allowed to quote the blasphemer verbatim, against the Rambam in 2 Avoda Zara 8, who says אזהרה של מגדף מנין שנאמר אלהים לא תקלל. בכל יום ויום בודקין את העדים בכינוים יכה יוסי את יוסי. נגמר הדין מוציאין את כל אדם לחוץ ושואלים את הגדול שבעדים ואומרים לו אמור מה ששמעת בפירוש והוא אומר והדיינים עומדים על רגליהם וקורעין ולא מאחין.
Eli sent us a mareh makom to the Orchos Chaim (not the Rosh: he was a talmid of the Rosh and his Sefer was the predecessor to the Kolbo) here, an amazing tzushtell to the Rogatchover, exactly on point and using the Tosfos in Sanhedrin:
This is also discussed in the Pachad Yitzchak (Lampronti) on Kedusha.
Rav Hirshprung points out that:
a. the Teshuvas HaGeonim in Tosfos in Sanhedrin shows that the shira of the Malachim does not take place on Shabbos.
b. The Teshuvas HaGeonim says that Klal Yisrael has a shira on Shabbos that takes the place of what the Malachim do during weekdays,
c. According to the Gemara in Chagiga and the Mahrsha, a whole week the Malachim are silent during the day in deference to Klal Yisrael and their Shira is only at night,
d. therefore, the only change that occurs on Shabbos must be at night- Maariv.
e. Since, as the Mahrsha says, the Malachim are silent only during our tefillos chova, then
f. the change in our Maariv must be from Reshus to Chova.
QED, Maariv is a reshus on weekdays but a chova on Shabbos.