In the last two decades \sim 200 quasars have been discovered at z > 6 , hosting active super-massive black holes with masses M _ { \bullet } \gtrsim 10 ^ { 9 } { M _ { \odot } } .
While these sources reflect only the tip of the iceberg of the black hole mass distribution , their detection challenges standard growth models .
The most massive z > 6 black hole that was inferred thus far ( J0100+2802 , M _ { \bullet } \approx 1.2 \times 10 ^ { 10 } { M _ { \odot } } ) was recently claimed to be lensed , with a magnification factor \mu = 450 .
Here we perform a consistency check of this claim , finding that the detection of such source requires a bright-end slope \beta \gtrsim 3.6 for the intrinsic quasar luminosity function , \Phi ( L ) \propto L ^ { - \beta } .
Commonly used values of \beta \sim 2.8 are rejected at > 3 \sigma .
If the claim is confirmed , it is very unlikely that all the remaining 51 sources in the SDSS sample are not magnified .
Furthermore , it suffices that \gtrsim 25 \% of the remaining sources are lensed for the intrinsic luminosity function to differ significantly ( i.e. , > 3 \sigma ) from the observed one .
The presence of additional extremely magnified sources in the sample would lower the requirement to \sim 4 \% .
Our results urge the community to perform more extended multi-wavelength searches targeting z > 6 lensed quasars , also among known samples .
This effort could vitally contribute to solve the open problem of the growth of the brightest z \sim 7 quasars .