Yes, sharding would kill performance under contention, which characterizes many OLTP workloads (e.g. top ten bestseller list on Black Friday, super stocks like NVIDIA, the big 4 banks on a switch, PhonePe/Google Pay on UPI etc)
Sharding is if anything a way to reduce the contention.
Transactions typically need to hit the same set of house accounts. For example, fee accounts, omnibus accounts etc.
Or a central bank switch may only have 4 major banks.
Or a system like UPI may have 85% of transactions flowing through 2 hubs. Say 50% through PhonePe. And say 35% through Google Pay.