This text is not proper German.
Weak rhyme Z - Alphabet
The name of the letter Z is pronounced as [t͡sɛt] (similar to "tset") and rhymes with the English words let, set, wet.
The German noun »Alphabet« is pronounced [alfaˈbeːt]. The last vowel is not [ɛ] but [e] which does not exist in standard English (but in many regional variations). These two vowels sound very similar, so for beginners it might be allowed to replace [e] by [ɛ]. There are even some German native speakers who do this, so this is acceptable. That's not the point.
The point is, that the vowel [ɛ] in »Z« is very short. This is the reason, why, when we write the full name of this letter, we write »Zett«. A double consonant always indicates a short pronunciation of the preceding vowel. But the last vowel in the German word »Alphabet« is pronounced long. This is not explicitely indicated in the normal spelling of the word, but when its pronunciation is written with IPA symbols, the symbol ː (which is not a colon) indicates stretched pronunciation of a vowel.
So, we have similar vowels but different lengths of this vowels. Both differences makes the rhyme weak, but still acceptable.
»das Sekret« means not »the secret« but »the secretion«, »the exudate«
»Sekret« is a German noun that refers to a substance produced by the cells or glands of an organism, such as sweat, tears, saliva, insulin and other hormones, but also nectar, resin, alkaloids, etc. in plants. This German word has never, under no circumstances, the same meaning as the English word »secret«.
»Sekret« is a false friend of the English word »secret«, as they look similar but have completely different meanings. While »Sekret« refers to biological secretions, »secret« in English refers to something that is hidden or concealed, often in the context of information or knowledge.
(One might object that knowing the alphabet is in fact neither a secretion nor an exudate, but that was not the intended meaning of this sentence.)
While the weak rhyme was acceptable, this is a very severe error that never should be printed in a textbook. Throw this textbook away. Do not trust it!
Wrong: »Ich kenne deutsches Alphabet«
This is also wrong and should not exist in a textbook.
You are right. »Alphabet« is neither a substance name (like Bier = beer) nor an abstract noun (like Musik = music) which both can go without article:
Ich trinke Bier. Ich höre Musik. Ich kenne deutsches Bier. Ich kenne deutsche Musik.
So, the sentence »Ich kenne deutsches Alphabet« is wrong. But still this is correct too:
Ich trinke ein Bier. Ich höre die Musik. Ich kenne ein deutsches Bier. Ich kenne die deutsche Musik.
You can use many substance namens and abstract nouns also with an article. But then the meaning changes. I will explain it with only the first of my examples. Similar arguments can explain the other sentences:
- Ich trinke Bier.
No article
This means, that I drink beer, like in English (»I drink beer«). This sentence does not tell which kind of beer and how many of it.
- Ich trinke ein Bier.
Indefinite article
This means, that I drink one glass of beer.
- Ich trinke das Bier.
This looks like an definite article but here the word »das« is not an article but a pronoun. So, you don't translate it as »the« but as »this« or »that«. This sentence means, that you either drink a specific kind of beer (like a specific brand) that is already known to the listener from the further conversation, but it can also mean that you trink a specific glass of beer. (There might be 5 glasses on the table, and you point with your finger to one of them while saying this sentence.)
