It's a bit confusing, because the basic abilities are obtained when a psyker has acquired a certain discipline... or what? I can't find a line that says he has to purchase anything with XP... does this mean he comes with all the basic abilities of each discipline or he has to purchase them with the skill "psychic technique", despite them having no price at all?
The rules here are a lot more complicated than they need to be.
To begin with, Psychic abilities are divided into Disciplines, Basic Techniques, and Techniques. Each Discipline has one Basic Technique, which you automatically acquire when you learn the discipline. Each Discipline has several Techniques.
You cannot learn a Technique unless you have first learned it's Discipline.
Page 157 tells you which disciplines you start with:
Gaining a new discipline is easy. You take the Psychic Discipline talent. This is available at Rank 3, and Rank 6 of Astropath Transcendent. You may not access more than three Disciplines.
Techniques get complicated. To start with, you purchase the Psychic Technique Talent. This allows you to buy a Technique which you meet the prerequisites for. What are the prerequisites? They're spelled out on page 159.
In order to take this Technique, you must know the Telepathy discipline, and have the Mind Link technique. You must also use a version of the Psychic Technique Talent which costs at least 300xp, which is not available until Rank 5.
Note that the Astropath always receives Psychic Technique (x2). So you get to choose two techniques each time you purchase it.
A psyker starts out knowing a Discipline (or Disciplines) depending on his career path; for example, from Psychic Techniques (Rogue Trader Core Rulebook, p 157)
In order to access another Discipline, they need to take the Psychic Discipline Talent (Talents, P 104):
For the Astropath, this is at Career Rank 6, so the Astropath can know two disciplines.
When you get a new Psychic Discipline by purchasing the Psychic Discipline Talent, you get the basic technique free. You always get the basic technique for free when you have access to a discipline, unless the GM houserules otherwise and provides an XP cost for it.
In order to access another Technique for any known discipline, they need to take the Psychic Technique Talent as you mention.