A soak at mid-500's °C can soften the glass to enable sealing against the mould.
The only glass that this might apply to is lead crystal.
The idea seems to be that it is possible to fire the glass in such a way that it conforms to the edge of the mould, so trapping air inside before the glass begins to slump. It goes on to indicate this is a cause of large bubbles from under the glass. This trapping of air is possible of course. But not at a temperature of ca. 550°C. Although this is above the strain point of fusing compatible glasses, it is not in the range where most glass begins to soften.
Bullseye, Spectrum, and Uroboros all only begin to bend near 600°C. This means there is no possibility of the glass conforming to the edge of the mould at 550°C before it begins to slump. The softening point of Float glass is around 720°C, and although with a large span, a 4mm piece will begin to bend at about 610°C (thicker glass will begin to bend earlier), it takes up to an hour for the glass to take up a gentle bend at that temperature.
It is possible that you can seal the edge of the glass against the mould in a side-fired kiln that is quickly fired. This is where the edge receives greater radiant heat than the top surface. However, I doubt that this will happen even in a side fired kiln until nearer 600°C.
All myths have an element of truth in them otherwise they would not persist.
They also persist because people listen to the “rules” rather than thinking about the principles and applying them. It is when you understand the principles that you can successfully break the “rules”.