Simple answer. Yes it is invulnerable sorta speak.
Being able to detect something and being able to block something are two completely different stories.
DS flashcards with reprogrammable FPGAs could always be detected, and Nintendo has tried to block them since day one by blocking certain game IDs and updating their blacklist, each time the card manufacturer would update the FPGA with new card ID negating Nintendo's block.
In short Sky3DS cannot be blocked because it can spoof the card ID on the fly, using the same principle as say a R4 Gold when it flashes its FPGA to bypass Nintendo blacklist, but Sky3DS does this on the fly.
Cards that don't require an exploit and use hardware card spoofing are unblockable, so long as they can change the card ID on the fly.
Nintendo would need to literally add 10000 card IDs into their blacklist just to block the Sky3DS for one day, because the very next day Sky3DS can update their template and bypass the blacklist by swapping out all their card IDs.
Although Sky3DS can be detected just like the R4 Gold or DSTwo, it can't be blocked in a reasonable fashion that cannot be bypassed, which is why Nintendo hasn't done it yet.
Also blocking 10000 card IDs can be very dangerous since some of those cards could be circulating in the used market, and you'd hurt real customers, which Nintendo can't risk.
Just my two cents.