I think he means that a doctor with 15 years experience after school can change hospitals they work at with minimal impact.
A hiring a senior engineer with 15 years experience, but not at your company, might not compare as favorably to an intermediate engineer with 8 years total experience, but 3 years at your company.
A hiring a senior engineer with 15 years experience, but not at your company, might not compare as favorably to an intermediate engineer with 8 years total experience, but 3 years at your company.