Is this just an error or am I missing something?

I suspect the intended meaning is, "As the farmers, we provide all our teas with a Tea Passport..."

The producers are saying that they have farmed the tea themselves, rather than bought it from others, and therefore can vouch for its "quality and sustainability".

I think this may be a case of a misplaced modifier.

Consider the part As the farmers, all our teas...

It seems to suggest that teas are the farmers.

Equivalently, if you use the participle phrase Being the farmers, all our teas..., the participle phrase Being the farmers is left dangling.

So, one might rephrase the sentence as—

As the farmers we sell all our teas with a tea passport, guaranteeing quality and sustainability from bush to cup.

As it stands this sentence is grammatical (with the added comma), but makes no sense (the farmers come with a Passport… and so do the teas.).

What was probably meant and what makes most sense is this.

• As we are also the farmers growing those teas, we are able to guarantee quality and sustainability from bush to cup and that is ascertained for each one of them in the Passport that comes with it.

I hear a mistranslation and a word that is missing, but assumed:

"As the farmers know, all our teas come with blah, blah, blah, but really."

As it stands, our farmers are dangling in wait of Lord-knows-what but a hook to hang their straw hat on.

Remember that marketing BS does not have to do more than put us in a good mood. So when all else fails, I drop my standards and say Charming. Hopefully, we can move on.

Consider that in the US our Truth-in-Advertising laws allow for evocative language such as soap that is 99 and 44/100% pure without factually assuring that percentage (since 1895).

When language becomes a slogan, it earns some leeway.

And that's 150% true.

The sentence is something akin to the passive voice: although grammatically speaking, the subject of the main verb is "all our teas", the teas aren't really performing the action, they are the recipient of the action. A more clearly passive voice construction would be "all our teas are sent with a Tea Passport guaranteeing quality and sustainability from bush to cup", and the active voice version of that would be "we send all our teas with a Tea Passport guaranteeing quality and sustainability from bush to cup". Then "As the farmers" modifies "we". So this is a case where the writer has two different factually equivalent, but not grammatically equivalent, phrasings in their head, and the write one down, while having the other in mind when they including the modifier at the beginning of the sentence.