This recommendation is probably for high intensity lamps on the order of 750-1K Watt where the intensity is quite high.
Rules of inverse squares, e.g. 400 PPFD at the canopy will dramatically increase by halving the distance. It follows 1/d^2.
For example, if the intensity at 18 inches is 400 W/m^2, the proportional intensity at a distance of 9 inches increases to 1600 W/m^2. That’s greater than what we receive at the earth’s surface from the sun (but it is also concentrated into a very small area). Yikes.
For the intensity we are talking about here, it is probably low enough that the plant could be right next to the bulbs. In that case, the larger concern would be the heat from the bulb.
But, in any case, if you see some bleaching or burn simply move the bulbs further away at that point.
Recent example of where I burned some leaves that were practically in contact with a relatively intense lamp:
Moved the lamp an additional several inches away and the problem was resolved. Easy enough. Until the plant grows into the lamp again, that is.
For very large changes in the intensity (e.g. moving a plant from indoors to the outdoors), it is best to acclimatize the plant to the new situation. The plant will respond to make efficient use of the available light. If the intensity of the light source is low, it will develop the ability to best utilize the available light. A sudden change in the intensity will overload the plant’s capability leading to light stress. I do not think that is a concern in this case but you can always move the new lamps closer daily, as well.