Yes. Radon enters buildings through cracks, joints, service penetrations and gaps in the floor construction, and concrete alone is not an effective radon barrier. That is why a dedicated radon-resisting membrane, fully sealed at all penetrations, is specified under Part C rather than relying on the slab itself.
Soil gas moves into buildings because the inside of a heated house is usually at slightly lower pressure than the ground beneath it. That pressure difference draws air through any available path: shrinkage cracks in the slab, the joint between floor and wall, gaps around pipes and cables, and openings around service ducts.
A concrete slab reduces but does not stop this flow. Effective protection depends on a continuous radon-resisting membrane properly lapped and sealed, combined where necessary with sub-floor depressurisation using a sump.