The character of the laws of shemitta as being pre-mishkanic and of a moment earlier than the building of the mishkan in the Sinaitic epiphany means, if properly understood and kept, that the laws of shemitta are prior to the consecratory character of the mishkan.
Shemitta is decidedly not qodesh because it belongs to an aspect of the covenant that pre-dates the covenant's consecration.
The clue can be found in the interstitial passage between Chapters 25 and 26 in Leviticus 26:1,2 where quite cryptically the miqrah speaks of not erecting a pessel or a matzevah of any elillim you prepare nor of placing in your lands a maskis stone to bow down to; and of yes guarding the shabbatot of God and of fearing His miqdashot.
This in contrast to the reference in parshat Qedoshim tihiyu (Leviticus 19:3) where He exhorts Israel to guard His shabbatot and fear their parents. There the exhortation is non-mishkanic but individual and not collective; here the exhortation is collective and pre-mishkanic.