I assume this is a mix of two things, one already known and one which may not be known.
1) Achievements have hardcoded requirements, which means even if you complete the achievement through some other means (boosting levels) the achievement will remain locked until you also complete those hardcoded requirements. - This is a known issue.
2) Prior to the Mining and Smiting Rework, the old Doric and Boric Tasks required level 75 smithing to make Adamant scimitars. Now, it only requires level 70 smithing with the new Doric and Boric tasks, but I assume they based the achievement on the old requirements.
Tagging u/JagexStu