That is for name only.
In reality there are no limits on Indian Brahmos.
This has been admitted even by Indian officials
Yes MCTR is non binding and even if it were a binding agreement, can't count on Brahmos being limited to 300km which it almost certainly is not. In fact Brahmos is the only credible offensive missile capability in Indian arsenal at the moment. Until India inducts Nirbhay in large numbers, air launched Brahmos is of the greatest threat to PLA positions and infrastructure.
Whether or not PLA air defence and EW can deal with Brahmos is unknown but there are a lot of things designed and fielded to do just that. Mach 3 to Mach 4 isn't difficult at all, particularly when some of those air defense systems are designed to intercept maneuvering MIRVs. interception of these things are mostly a matter of range and speed, most of the time they are just not in position to intercept fast moving targets where speed and range are a part of the same calculus. Air defence systems positioned next to the targets they're defending gives them a much greater interception probability which would be the case for Brahmos attacks. Brahmos is also a large missile, not all that fast (mach 3 isn't fast anymore), not stealthy in the slightest, and does not have unpredictable flight profiles - non terrain hugging and does not avoid radar or circle around targets like many modern cruise missiles are capable of. In fact IAF SCALP missiles will be a far more troublesome thing but they are still being inducted and trained on.
I'm sure PLA would have had plenty of practice against intercepting YJ-91 and YJ-12 missiles which are mach 3 to mach 4.5 missiles and do terminal phase maneuvering where they use the trajectory that continues to adjust interceptor missile interception calculations to continue dragging the missile around for optimal effectiveness. Hard to explain without visually showing the maths involved but think atmospheric pressure variation, KE-PE tradeoffs, and missile interception point calculations constantly being changed by maneuvering missile. The maneuvering missile trade a tiny bit of that mach 4 energy to reduce the energy of the interceptor and its range very dramatically.
YJ-91 is much smaller than Brahmos so harder to intercept and if they can intercept that with close to 100% reliability, they can almost definitely intercept Brahmos which has 1990s based flight control software. Maybe they've upgraded it but is unlikely to be much better than YJ-12 or YJ-91. Those old Moskits were too easy to defeat even back in the 2000s for PLAN. They realised the USN will have no issue whatsoever.
PLA have probably also done a lot of practice against CX-1 (even weaker missile than the above) and HD-1. Both of those were built for a random few export customers but are basically mock missiles for interception practice.
While India has Brahmos as its current top end offensive non-ballistic missile, PLA has YJ-91, YJ-12 (anti-surface sub-variants are basically confirmed in Chinese literature), DF-100, rocket boosted HGVs, and air launched ballistic missiles. Not that it needs to use any of those with such artillery advantage.
Meanwhile there is little indication Nirbhay is satisfactory or anywhere near approved for induction. Russia isn't trusting enough to sell India even watered down versions of its Zircon anti-surface missiles since India showed off the Akula interior to the American visitors. Also it's too strategic a Russian weapon to sell to India given India's flirting with the US. So Brahmos II is even further away than Nirbhay. SCALP will take a few more months at least before sufficient operational training and tactics developed and enough are delivered from France.
