> We used Cox proportional hazards models to estimate hazard ratios... between self-reported hearing aid use status (hearing loss with or without hearing aids) at baseline and risk of dementia

The top-level article discusses a randomized controlled study and therefore can make a much stronger claim of causality:

> The researchers randomly assigned participants to one of two interventions. About half received hearing aids and instruction in how to use them. The other half were assigned to a health education program focused on promoting healthy aging. Both groups received follow-up visits every six months to reinforce the training.