Without knowing where you got your information it's impossible to explain the precise reasoning used by your source, but the most likely scenario is that the source has taken the linguistic group to which their language may have belonged -- possibly, conjecturally, the Germanic group of languages -- and applied it to their ethnic group.

This is a category error, but a very common one. You'll often hear people talking about 'Indo-European people', for example. It creates misunderstandings and misapprehensions. Sometimes it just comes from a failure to distinguish language and ethnicity, sometimes it's a shortcut for saying 'an ethnic group whose language belongs to X group', sometimes it's a malicious tactic for sneaking in nationalism. Regardless of the intent, I find it always has the effect of sneaking in nationalism.

As for 'Germanic': it doesn't mean 'German'. It refers to a group of languages associated with several regions in northern Europe, of which German happens to be an example. Modern Germanic languages include Swedish, Norwegian, Danish, Icelandic, Faroese, Dutch, Frisian, and English, as well as German. Several of these are more a cluster of related languages, than a single language).

You are thinking that "Germanic" pertains to what is now the country of Germany, but this term is in fact linguistic and cultural and applies in antiquity to the group(s) which spoke Germanic languages and inhabited a vast region of northern, central, and eastern Europe in the first six centuries CE, including Scandinavia. We know that the Heruli (Greek Erouloi) spoke a Germanic dialect, probably an East Germanic language (but possibly North Germanic), and from the personal names which have been recorded from among them, many of which are very similar to Gothic personal names. The Germanic peoples were one branch of the Indo-European family tree, and dwelt in northern Europe in antiquity alongside other branches of IE, like the Celtic speakers, Italic speakers, Hellenic speakers, and, later, the Baltic and Slavic speakers. During Roman Imperial times, the Heruli seem to have dwelt in the area of Sjælland, probably around where modern-day Copenhagen and Malmo now sit. Some others believe the Heruli might have come from the Middle Danube region. In either case, the Heruli did not leave from Sjælland and march directly into Greece. They were part of a larger migration of Germanic peoples into eastern Europe, especially in this case to the Black Sea. There, these groups, including Heruli and Goths (if they are indeed distinct), took control of Graeco-Roman cities on the coasts of the Black Sea and used their ships to launch attacks on the Balkans/Greece dated 267/268 and 269/270 CE. It is this event that you have probably read about.

The Heruli are specifically mentioned among this marauding group for their attack on Athens itself, where they were driven away by the famous efforts of Dexippus. Later a Heruli chief called Naulobatus was honored by the Romans and even became a high-ranking official. The best modern source for them is probably Roland Steinacher, who has written quite a bit on this and related topics.

The Heruli are something of folk heroes in modern times, especially in northern Europe. They played a rather small but famous part in the chaotic times of the end of antiquity, and are often left off of general maps of movements of Germanic peoples. They are often lumped together with Goths on maps of movements. There is an old theory that they were some kind of elite mercenary band, but there is no good evidence for this.