Post proof that exercise variety is detrimental to achieving lifting goals. I'll concede that if you are competing in a strength sport, it is best to focus on the movements in that sport.
They’re likely just meaning it makes it harder to progressive overload. If you stick to a programme, you know it’s just about modifying weight, reps, rest, time under tension etc. But if you are changing exercises constantly, it can be difficult to know what weight or rep range will get the right results from that exercise since it’s not really comparable to another exercise.
Progress will _only_ be made with sufficient stimulus, and your body will rapidly adapt so the stimulus is not sufficient -- unless you do progressive overload (and ideally occasional deloads too)
