shirk is of 2 types: akbar (polytheism) and asghar (not polytheism).
Shirk akbar will mean a person is no longer muslim.
Shirk asghar in most situations will mean the person has sinned but is still a Muslim. The Prophet (saw) said:
وَعَنْ مَحْمُودِ بْنِ لَبِيدٍ - رضى الله عنه - قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ إِنَّ أَخْوَفَ مَا أَخَافُ عَلَيْكُمْ اَلشِّرْكُ اَلْأَصْغَرُ: اَلرِّيَاءُ } أَخْرَجَهُ أَحْمَدُ بِسَنَدٍ حَسَنٍ. (1946) .
1 - حسن. رواه أحمد (5 / 428 و 429) وزاد:" يقول الله -عز وجل- لهم يوم القيامة -إذا جزى الناس بأعمالهم-: اذهبوا إلى الذين كنتم تراءون في الدنيا فانظروا هل تجدون عندهم جزاء".
Mahmud bin Labid (RAA) narrated that the Messenger of Allah (ﷺ) said:
“The thing I fear most for you is the lesser shirk, which is showing-off (of good deeds).” Related by Ahmad with a good chain of narrators.(1946) and in another narration: On Judgement Day, as people are being rewarded for their deeds, these people will be told: "Go to those people in front of whom you were showing off your good deeds - and have a look - are they going to reward you for it? (and obviously they won't reward them).
Also, the word shirk does not always mean Polytheism. It literally means to share something, to be a partner in something. A شركة is a company, because it is owned by many people. the phrase بِيعَ شِرْكٌ مِنْ دَارِهِ means a شرک, or share, of his house was sold. Thus, in your textbook, when it talks about 3.3) To fear someone more than Allah, it use of the term shirk here, means the lesser shirk, since you should be fearful of Allah alone, and not fearful of men. HOwever, fear is a natural feeling, and it would not be a sin if the fear was from the harm of another person.
for example, even Musa (as) was scared of Firawn and his guards: فَأَصْبَحَ فِي الْمَدِينَةِ خَائِفًا يَتَرَقَّبُ (Musa (as) spent the morning in fear, and on the lookout (for Pharoah's guards, in case they came to arrest him). (28:18)
The believers are told that if they are scared of the enemy attacking them, they should pray riding or walking فَإِنْ خِفْتُمْ فَرِجَالًا أَوْ رُكْبَانًا 2:239