From my understanding, it's not just the issue of how far the BrahMos can be detected. It's also the issue of how well the radar can track - and keep a lock on - a supersonic target. If the radar can't keep a lock on the target, then a semi-active radar-homing (SARH) SAM might not be appropriate. There's also the issue of the SAM's speed. The LY-80N's speed isn't clear, but
Army Recognition claims to have it, and the LY-80 is 300 meters/second, i.e. Mach 0.87.
I'm unsure if the LY-80N has a quasi-missile defence system wherein it can determine an optimal intercept point, but I doubt it. The trend with quasi missile defence systems is that the missile has an active terminal seeker (enabling the radar to send the SAM to a specific point, after which the terminal seeker kicks-in). These SAMs are also much faster - e.g. Aster 30 is Mach 4.5, HHQ-9 is Mach 4.2, SM-6 is Mach 3.5, etc. Basically, no one except Pakistan is banking on the use of a SARH sub-sonic medium-range SAM for intercepting a supersonic AShM.
Not BrahMos, but the thing carrying BrahMos (i.e. Su-30MKI, possibly MiG-29KUB, Rafale/Super Bug later on, etc).