That was true in the hot chassis tube radio era as a half wave rectifier tube could supply enough current direct from mains, and the cost of implementing a bridge rectifier was prohibitive but transistorised and later integrated circuit hot chassis TVs typically had a bridge rectifier feeding the DC bus, making the chassis hot with half-wave rectified mains no matter which way round you connected the power plug.
There were also oddities like Salora's Ipsalo II which omitted the usual switching transformer, and instead had a line output stage with an isolated primary, so the line output stage and line scan coils were 'hot' but most of the rest of the circuit wasn't
Working on such equipment was a specialised trade requiring extensive training, usually formally taught at a trade school or by one-to-one mentoring. The bench setup was quite particular as well, mostly in terms of removing as many grounds as possible from the bench and nearby area, and minimising the risk of contact with those that couldn't safely be removed or insulated (e.g. the CRO). The problems with learning it from the internet are: finding reliable information in the first place that isn't either dangerously out of date or dangerously wrong; and also you've got nobody watching your back and correcting you if you start to develop dangerous habits.