Well, that paper is suggesting that the hyervelocity stars have to be ejected from close binaries disrupted by a SMBH.

The velocity of such stars are ~ 600 km.sec^{-1} whereas typical galactic orbital velocities < 300 km.sec^{-1}, so they are travelling at twice this rate. That paper refers to one star with a velocity in galactic coordinates of +853 ± 12 km.sec^{-1}, which is, at its location, twice the velocity of escape from the galaxy.

Now they say for a star to retain an escaped velocity of this magnitude the initial velocity of ejection must be > 1000 km.sec^{-1}, which is derived from the Kelperian escape velocity of
[itex]v[/itex] ≈ [tex]\sqrt {\frac{2GM_{BH}}{r}}[/tex] > 1000 km.sec^{-1}

As G ~ 7 x 10^{-8} c.g.s units and they suggest
r < 0.01 parsec ~ 3 x 10^{16} cms. and as
1000 km.sec^{-1} is 10^{8} cm.sec^{-1} and M_{Solar} ~ 2 x 10^{33} gms.

then M_{BH} ~ 10^{16}[tex]\frac{r}{2G}[/tex] ~ 10^{6}M_{Solar} .

However r ~ 0.01 parsec ~ 10^{3} AU and a BH could approach much closer than that.

If the binary system were approached to 1 AU then the BH would only have to have a mass of 10^{3}M_{Solar}.

In other words an IMBH would do, rather than a SMBH, and there may be many more of them.