This is a fine process for what you are doing because you use such a small amount of soured wort in a batch; however, it is a bad beer waiting to happen at a larger scale. Using luck of the draw to culture off grain increases the probability of off flavors in the beer because the volume of off flavor compounds is far greater in a full batch soured in this method over five percent or less that you use. That's not to say one cannot ever get a good sour beer out of this process but the probability is not great. That's why most brewers kettle souring use selected lactobacillus strains.
That is not my finding. When properly acidified to under 4.5 and kept anaerobic, the potential to develop off-flavored or spoiling organisms is exceedingly small. In addition, when creating a starter culture this way, you have the ultimate QC equipment in the form of your nose and palate to check the result prior to use.
And the risk of having some yeast or other organism in your soured wort is also not a concern if you are using this wort in typical pre-boil settings. That soured wort will be fully sterilized via the boil.
My experience with pure lacto strains is that they have narrow flavor profiles that are not pleasing in finished beers. Using the handful of grain inoculation is an effective way to create a flavorful and safe starter.