The first thing to realise here is that that is not Old English. Read the quote carefully:
an *-ian verb-forming suffix in Germanic
That means the form is Proto-Germanic, rather than Old English. It’s perfectly customary – in fact it’s the standard convention – to use asterisks to indicate that a word or form is reconstructed, meaning that there is no textual attestation of it, but it can be inferred based on comparative evidence that this is what it must have looked like.
In the case of Proto-Germanic, the entire language is unattested (because the speakers of the language had no writing system – writing arrived in Germanic areas much later, when the proto-language had already developed into several different languages), so any form that is said to have been used in Proto-Germanic should have an asterisk in front of it. But asterisks are also used to indicate single words or forms from attested languages, if the word itself happens not to be attested. For example, there are a number of verbs in Latin which are only attested with various prefixes; the base verb itself is not seen in any extant Latin texts. Even though it’s often fairly easy to guess what the base form would be, we use an asterisk to show that it is, after all, an educated guess and not based on actually reading the word in a text written by a Latin speaker of the time.
Note that asterisks are also used to indicate that a form is impossible (e.g., to illustrate that the Latinate prefix in- always assimilates to certain following consonants, you could say that *inlegal and *inpossible don’t exist). This is an entirely separate use which does not apply here, but which is worth knowing.
