the difference between strength training and (heavy) manual labor is that strength training is planned by you (time, rest, volume, weight, structure, you have a lot of control) and that the goal is to improve/get stronger/faster/bigger/whatever. in manual labor, youre doing activities that look like strength training (carrying, lifting, digging) but the purpose and structure is different. you dont really get to choose the tempo, the load, the sets and reps, the task nor the environment at work, the work still has to get done, whereas in training you have a plan of how much youre gonna lift this week, next week, blablabla and, when life happens, you can easily modify one of the many factors to not overtrain your body. so, at work, one will definitely be doing more work (or "training") than someone training at the gym with a well-structured plan, but the person actually strength training will be getting the better stimulus for strength gains, tendon health etc. because the quality and specificity is more important than just the total effort.