This is still a more dependable approach with resource constraints. Fragmentation is eliminated and you can monitor pools for usage in a worst case scenario. The only other risk here versus true static allocation is a memory leak which can be guarded against with suitable modern language design.

LwIPs buffers get passed around across interrupt handler boundaries in and out of various queues. That's that makes it hard to reason about. The allocation strategy is still sound when you can't risk using a heap.