Request coalescing in a shared cache does not solve thundering herd, it just reduces propagation to backend services. Your cache is still subject to a thundering herd, and may be unable to keep up.

The only way to solve thundering herd - which is that a load of all requests arrive within a short timespan - is to distribute requests over larger timespan.

Reducing your herd size by having fewer requests does not solve thundering herd, but may make it bearable.