@LGenzelis, I followed up in the there topic with an answer here.
To summarize, the webhook for MRs will be “triggered when a merge request is created/updated/merged”. There isn’t an option for just “merged” and this setting is at GitLab, not Netlify, so we cannot change this behavior. It might be helpful to file a feature request at GitLab asking for more fine grained control of the webhooks there.