Language like this is often used in the Mishna, Gemara, and other halachic works. Here are a few examples:
אמר שמואל כל היושב בתענית נקרא חוטא
Shmuel said: Whoever sits in observance of a fast is called a sinner.
SA YD 203:1:
אל תהי רגיל בנדרים כל הנודר אף ע"פ שמקיימו נקרא רשע ונקרא חוטא:
Do not become accustomed to making vows. Anyone who vows, even if they keep them, is called evil and is called a sinner.
וְאָמַר רַבָּה בַּר חִינָּנָא סָבָא מִשְּׁמֵיהּ דְּרַב כֹּל שֶׁאֶפְשָׁר לוֹ לְבַקֵּשׁ רַחֲמִים עַל חֲבֵירוֹ וְאֵינוֹ מְבַקֵּשׁ נִקְרָא חוֹטֵא
And Rabba bar Ḥinnana Sava said in the name of Rav: Anyone who can ask for mercy on behalf of another, and does not ask is called a sinner.
What do all of these examples have in common? It's that the thing they're describing which causes someone to be "called a sinner" isn't really a sin. There is absolutely nothing in halacha that prevents a person from taking fasts upon himself. And there is no mitzvah among the 613 mitzvot in the Torah that says you must ask for mercy on behalf of everybody you can. Making vows is also not a sin on its own. In fact, just 5 seifim after the Shulchan Arukh says that someone who makes vows is "called evil" and "called a sinner," he gives some examples where making a vow is recommended:
האומר אשנה פרק זה וירא שמא יתרשל בדבר שרי ליה למנדר לזרוזי נפשיה וכן אם ירא שיתקפו יצרו ויעבור על איזו מצוה ממצות לא תעשה או יתרשל מקיום מצות עשה מצוה לישבע ולנדור כדי לזרז עצמו:
A person who says "I will learn this perek, and is afraid lest they become negligent in this matter, it is appropriate for them to vow in order to quicken themselves. And so, if one is afraid that their [evil] inclination will attack them and they will transgress any commandment of the negative commandments, or become negligent in the fulfilment of a positive commandment, it is a commandment for that person to vow in order to quicken themself.
These are all examples of rabbis explaining what they believe is the proper way to act. Not following these directions is an expression of poor midos, and perhaps Hashem looks upon a person who does these things the same way he would look upon a sinner. But they're not formal halacha.