It means the information from old video tutorials can lead to making the problem worse or end up softbricking the 3DS firmware. What might work in a specific scenario or firmware version might not hold true for a different conditions.I have been hearing people saying that and never told me what it means
Video tutorials are often the bane of 3DS homebrew community because this heaps more effort and troubleshooting undoing the damage that would have been avoided if the person hacking or modding their system had instead used the official or updated guides.