As explained in the article , verbs that follow prepositions are usually gerund forms that act as nouns instead of verbs(usually Verb+ing). But in the sentence: "Tell John to bring some food", 'bring' is clearly a verb and not a noun. So, which part of speech is 'to' in this context?
When "to" is used to accompany the infinitive form of a verb, it is known as the "infinitive particle" or "infinitive marker" (or in the documentation of some part-of-speech tagging software, simply "infinitive to").
So, for example, take the following sentence:
I went to the store to buy some food.
The first "to" is a preposition. The second is an infinitive particle that accompanies the infinitive verb "buy".