Is obrigado used in case of unclear gender of the author?
Can I deduce that the writer of the text is male or is there some kind of neutral male default that might be in use here?
The latter; male is used as default for neutral in Portuguese.
In Portuguese, words that can have either of feminine or masculine termination (e.g. padeiro/padeira, réu/ré) default to masculine if the gender of the subject is unknown or irrelevant.
As such, when sending a personal letter, you would write obrigada if you were a woman and obrigado if you were a man.
But things are different when attribution is irrelevant, unknown, or, in this case, the author is writing in name of an entity (company, foundation, organization etc.). In these cases, one defaults to male terminations.
See, for example, the Código de Processo Penal (Criminal Procedure Law) of Brazil:
No caso de requerimento do Ministério Público, do querelante ou do réu, o juiz mandará autuá-lo em apartado, (...)
As you might be wondering, this represents an enormous headache for the proponents of neutral language. In English, change he and she for they and you are pretty much done, it is actually grammatically correct. In Portuguese, there is no neutral pronoun, and one would need to introduce not only a new pronoun in the language, but also alter the termination of most nouns (epicenos, sobrecomuns or comuns-de-dois are exceptions), articles and adjectives out there.
And I want to emphasize that the neutral variants, such as proposed in the non-binary wiki (todes, ile), are neither recognized by the normative grammar nor by the vast majority of the population. Communication will certainly be impaired if those are used, and their adopters are often subject of disdain and even animosity.
Instead, some try to circumvent gender by picking words that both are consolidated in the language and convey neutrality, such as pessoa and quem in this document from the Superior Electoral Court of Brazil.
See also: Gender neutrality in Portuguese.