They're...both right. Sort of. How the term is used depends on which source you're looking at.

The Scythians were a real ethnic group with their own language and culture. They, the Sarmatians (later the Alans), and the Saka were all Iranian steppe nomads speaking eastern Iranian languages. The Scythians were the first of the group encountered by the Greeks when they started spreading into what would later become their Bosporan colonies because they were geographically closer to Greece than the other two. Even though 'Scythian' was originally applied to a specific ethnic group, it was later used to describe all Iranian steppe nomads due to lack of knowledge about their differences.

Nowadays, it's preferred to confine 'Scythian' to the actual ethnic group/culture, but a lot of older sources used it in the broader sense, so keep that in mind when you're reading about them.

The term Scythian has a similar use as the term Greek, in that it definitely describes a certain ethnic group but that ethnic group covers quite a wide variety of different tribes and people with different individual cultures but many similarities in customs and lifestyle.

Herodotus spends quite a large part of the Histories describing Scythian groups and spends some brief time listing different tribes and the differences he understands to exist between them. From his telling at least, each tribe was fairly independent but the more powerful amongst them, the Royal Scythians, considered themselves the true leaders of the steppe nomads.

It seems they were disparate enough that uniting them was difficult, but they were capable of joining together when threatened by an outside force.