Confused between Past continuous and Past perfect continuous

In these following sentences, which one is grammatically correct?

1. They were playing football since 10 o'clock.

2. They had been playing football since 10 o'clock.

According to this reference link:

Past Perfect Continuous Tense is used to express a continued or ongoing action that started in past and is continued until sometime in past. There will always a time reference like – for a few days, for ten months, for 2 years, since Wednesday, since 2011, since January etc. If there is no time reference, then it is not a Past perfect continuous tense. Without time reference, it is Past Continuous Tense.

But I saw the following example, which has used with time reference.

So, I am confused between past continuous and past perfect continuous. Please someone help me.

Thanks in advance.

(2) is correct. (1) is not idiomatic, whatever the book says. You can say "They have been playing since 10 o'clock" (and are still playing), but in the past tense it would have to be "They were playing football from 10 o'clock onwards" or "...from 10 until 12".