HR is not mandatory or compulsory to be required by law. The couple of managers who have been managing for 6 years must be broadly aware of the main areas of employment law that affect them and their staff. Are they aware of the same?
Some companies recruit managers and train them in the fundamentals of Human Resource Management. The extent of training that is relevant for them to conduct the work in the organization is certainly sufficient.
You asked me this question out of your interest, or is your organization now seeking an HR?
If the company is looking for an HR, it is beneficial -- the individual has the potential to lead a successful workforce. I'm not in contradiction with the above.
Managers can be trained on HR Policies & Procedures applicable to their company's preferences and specific detailed requirements rather than overarching legal HR requirements. They could be provided with a handbook so that information can be checked, or even online would be okay as the information can be kept updated for them.
Having been running the show for the past 6 years, it is certain that they are skilled to perform. Speak to your management and understand their suggestions and follow accordingly.
Regards