Not true at all. Judo and BJJ gis are strong in order that they can be used repeatedly without breaking, not so that the practitioner becomes reliant upon the strength of them. A person in a t-shirt and shorts still presents multiple handles that can be used against them, as well as having a handy garotte around their neck (much more effective for choking than a thick inflexible gi). In fact ripping weak clothing and bunching it is in some ways better than a purpose made gi in that it is painful and restricts movement much more effectively. Weak clothes can be ripped intentionally and used to rag someone around while restricting arm movement and ability to respond.
The other great thing about training in clothing is that methods which don't use the clothing (wrestling takedowns and controls, various submissions), need to be made to work with the clothing on, which is much more difficult. In this way applying them while not wearing bulky clothing becomes much easier.
A third way in which clothing helps is that it encourages a different way of using the body, encourages better hips, more use of legs, less likely to be square and double weighted. Wrestlers in gis are in many ways very easy to throw in a gi because they generally lack the hips and leg usage. Interestingly wrestlers with this usually come from countries like Georgia, Khazakstan and other ex soviet republics, where they have jacket wrestling folk styles.
Lastly training in a jacket produces an understanding of grips, grip fighting and grip breaking that is impossible to get in any other way. With this knowledge it is remarkably easy to stall, frustrate and beat larger, stronger, more athletic and more skilled no-gi wrestlers. Hard to understand why someone wouldn't want to learn to use this knowledge offensively, to understand how to break and overcome cloth gripping defence (which people do naturally when in a defensive grappling situation), and to understand how to use grips to frustrate and win against better opposition defensively?
To sum up, training in the gi is essential as it provides several benefits not available without gi. Most serious grapplers train both gi and no gi, which is the most sensible and comprehensive approach, gaining the benefits of the gi and maintaining familiarity with no gi.