Catalyzes the hydrolysis of 4-amino-2-methyl-5-hydroxymethylpyrimidine pyrophosphate (HMP-PP) to 4-amino-2-methyl-5-hydroxymethylpyrimidine phosphate (HMP-P). Belongs to the HAD-like hydrolase superfamily. Cof family. Could be the product of a pseudogene. It is N-terminaly truncated compared to othologs.