-IZE: unstressed (though strong)

According to the Longman Pronunciation Dictionary

-IZE /aɪz/: This suffix is unstressed (though strong) in Received Pronunciation and General American, but sometimes stressed in other varieties".

However I do not understand why the author contrasts stress and "strength" in this specific suffix.

The strong pronunciation of "i" is as in "file" and "line" and also as in "stick", "fit", …; the weak pronunciation is as in "albeit", "audio", and also "admiral" (schwa). That is all there is to it; that is what strength means; look up the first page for i in this dictionary and you'll find the same information.

The contrast is due to the fact that there is a tendency for the strong pronunciation to bear stress, but it is just a tendency.

Here is a link where explanations can be found concerning the meaning of "weak" and "strong" in phonetics: https://multimedia-english.com/phonetics/weak-vs-strong-forms.

John Wells, the author of the Longman Pronunciation Dictionary, uses a definition of stress that treats the primary stressed syllable in a word as the rightmost stressed syllable. This means that in words like digitize and utilize, where the primary stress is on the first syllable, neither of the two following syllables has any stress: ˈdigitize and ˈutilize. (This contrasts with the tradition of American dictionaries which tend to mark the final syllables of these words with a minor stress mark: ˈdigi​ˌtize and ˈuti​ˌlize.) For more on the concept of primary stress, secondary stress and their positions, see my posts here:

• Why isn't the T in “relative” flapped?
• What's the lowered “single quote” lookalike marking in phonetic symbols

However, the second and third syllables of utilize and digitize behave differently in terms of certain affects on the adjacent sounds: in many accents of American English, the "t" in utilize may be voiced and "flapped/tapped" (roughly, [ˈjuɾəlaɪz]) while the "t" in digitize may not be (roughly, [ˈdɪdʒətaɪz]).

Wells' explanation of the difference is that t is flapped in American English only before "weak" vowels, not before "strong" vowels.

Incidentally, I'm not sure that Wells is correct in his formulation of the t-flapping rule. Subjectively, flapping seems possible to me (I'm an American English speaker) in the edge case of Latin-style plurals in /aɪ/ in words where the singular ends in [ɾəs]: emeritus [əˈmerəɾəs], emeriti [əˈmerətai], ?[əˈmerəɾai] or stratus [ˈstreɾəs], strati ?[ˈstreɾai]. However, my intuition that [ɾai] is possible here is not supported by dictionaries such as Merriam Webster and the American Heritage Dictionary, which transcribe the last syllable of emeriti with a minor stress mark.

In any case, Wells uses the "strong vowel"/"weak vowel" concept not only to explain t-flapping, but also other things like a vowel's tendency to have a reduced pronunciation.