To be fair, this is a Pondicherry-class minesweeper only 60m long and less than 900 tons full load.
One suspects that the reason behind the additional reflectors is that they aren't looking to evaluate the performance of the seeker, but rather to evaluate impact/damage characteristics. A "miss" would fail to achieve test objectives and require another missile to be used, in turn requiring reallocation from inventory and ultimately drawing from expense accounts, which could have career implications. From a broader perspective, this is obviously silly: if BrahMos' seeker has trouble with smaller warships, that absolutely needs to be identified and addressed. But from the perspective of mid-level bureaucracy, I can see the motivation to control extraneous factors.
I think you will find that such tight control of supposed "real world" tests is quite common in military services worldwide, and fear of failure and important people looking silly or incompetent is the biggest reason why. Most tests and exercises are designed to "prove" or "demonstrate" something that has already decided upon, and nobody involved wants to learn that e.g. the adversary force has its own ideas about how it is going to operate. This is why officers at all levels need to be empowered to take initiative and make mistakes without fear for their career prospects.
See: USMC General Paul van Riper and the exercise
. Playing a "Red Team" (basically a thinly veiled Iran), he used small missile craft to sink most of a USN carrier battle group only to have the exercise paused and USN's ships "refloated" because that wasn't the way the exercise was meant to be going.