Your own quote. He's referring to the present threat.
The "sages' seal" is not the "seal of the gods", sir. If that was his intention, he would not have referred to him as being sealed away "by the very power he hoped to command" (the power of the gods), but as being sealed away by the hero, as the legends say.
Let's parse sentences, shall we:
I do not know why the seal [that] the gods [made] has failed...I do not know why the [Sage's] seal [that] the gods [blessed] has failed...Metaphor runs rampant. Show than the second interpretation is invalid, and since you're attempting to disprove an alternate interpretation, you'll need to use a different quote to do so.

Remember, Ganon had already escaped the Sacred Realm long, long before he began to threaten the Great Sea. Hundreds of years, in fact. And once he escaped his prison below the waves (the seal the king would have to be referring to), then he managed to become a threat once again.
Let's ponder this.
(1) The Sages sealed Ganondorf within the Sacred Realm. This is known from Rauru's quote: "The Sages' Seals will contain all the evil power in the void of the Realm..."
(2) Ganondorf escaped the Sacred Realm in
Wind Waker. (I certainly hope I don't have to prove this.)
Now, let us investigate the word "seal" as used by
Ocarina of Time to divine a meaning for it.
ZELDA
But the entrance [to the Sacred Realm] is sealed with a stone wall called the Door of Time.
GORON IN FRONT OF DODONGO'S CAVERN
On top of that, a Gerudo in black armor used his magic to seal the entrance with that boulder!
DARUNIA
Nothing has made me happier than helping you seal the evil [of the Fire Temple] here!
SHEIK
With one exception, the Zoras are all now sealed under this thick ice sheet...
SHEIK
Impa, the leader of Kakariko Village, had sealed the evil shadow spirit in the bottom of the well.... But the force of the evil spirit got so strong, the seal of the well broke, and it escaped into the world!!
ZELDA
After you opened the door of time, the Master Sword sealed you away in the Sacred Realm...Each of these quotes include the word "seal," and over them all we can come up with the intended connotation. Each of these talks about some sort of barrier--whether it be physical or metaphorical--to keep something separate from something else. The Sacred Realm was kept separate from Hyrule, Dodongo's Cavern was kept separate from the Gorons, the evil of the Fire Temple was kept separate from the rest of the world, the Zoras were kept separate (contained from) the rest of the world, Bongo Bongo was kept separate from Kakariko, and Link was kept separate from the flow of time in Hyrule. All of these references have a common thread. Therefore:
(3) Because of (1) and the intended connotation of the word "seal," the Sage's seal was intended to keep Ganondorf from Hyrule.
But we know that didn't happen forever. However it happened, Ganondorf broke out of the Sacred Realm to attempt to conquer the world. Certainly he didn't have his power because of the Master Sword--indeed, some other seal altogether apparently, he got out. This violates what the intention of the seal was; Zelda even mentions herself that the seal isn't going to hold forever: "Thanks to you, Ganondorf has been sealed inside the Evil Realm! Thus, peace will once again reign in this world...
for a time" (emphasis mine). Thus, there is an easy conclusion:
(4) Because (2) opposes (3), it is obvious that the seal mentioned within (3) failed.
Of course, you can look at this in two ways:
(5a) Ganondorf physically broke the seal.
(5b) Ganondorf found a way around the seal, thus making it seal imperfect.
Logically speaking, at least one of (5a) and (5b) must be true.
You, so far, have shown a case for (5b). So let's add that to the court record.
(6) (5b) is possibly true.
However, (5b) being true says nothing about (5a) since they're are not mutually exclusive statements; all that is guaranteed is that at least one is true. Thus, we give the benefit of the doubt to (5a) since it has not been disproven.
(7) (5a) is possibly true.
If (5a) is true, then (5b) need not be true. So,
(8) By (7) and (5), (5b) need not be true.
(9) By (8), (5b) is not fact.
Therefore, as of present, you're using interpretive spin and guestimation to say that you cannot be proven wrong. But that doesn't mean you've gotten a proof to prove anyone else wrong either.