Nucleotide sugar hydrolase that catalyzes the fourth step in the biosynthesis of pseudaminic acid, a sialic-acid-like sugar that is used to modify flagellin. Mediates the removal of UDP from C-1 of UDP-2,4-diacetamido-2,4,6-trideoxy-beta-L-altropyranose forming 2,4-diacetamido-2,4,6-trideoxy-beta-L-altropyranose (By similarity). H2O + UDP-2,4-diacetamido-2,4,6-trideoxy-beta-L-altrose = 2,4-diacetamido-2,4,6-trideoxy-beta-L-altrose + H(+) + UDP Monomer. Belongs to the PseG family.