Why is the nitrile group not hydrolyzed as well during hydrolysis of imine in Thorpe reaction?
It has to do with hybridization. The nitrogen in a nitrile is sp hybridized; imine sp2 and ammonia sp3. The more s-character in hybrid orbitals, the less basic the electron pair (protonated nitrile pKa ~ -10; pKb ~24). Ammonia has the lowest percent s-character (25%, NH4+ pKa 9.25). Imines lie in between although most examples you will find are measured in DMSO and are loaded with biasing phenyl groups.
Compare the carbon series: acetylene (sp, pKa ~25), ethylene (sp2, ~44), methane (sp3, pKa ~50). The closer the electron density is to carbon, the more acidic the C-H bond. If electrons are held closer to carbon, then it is more difficult for homolytic cleavage of a C-H bond. That is, acetylene is willing to give up a proton but not a hydrogen atom. Thus, Bond Dissociation Energy runs in the opposite order as acidity.

The harder it is to protonate, the harder it is to hydrolyze.

There were no conditions provided by user 481311 for the hydrolysis. Imines, such as the acetaldehyde methylimine (CH3CH=NCH3), will hydrolyze in water. As the User illustrates, imines hydrolyze readily in acid medium. These hydrolyses may be conducted at ambient temperature or at mild temperatures. Nitriles, owing to their low basicity, require more vigorous conditions like refluxing aqueous acid. Its all about conditions. I hope this helps.