If an early Christian author quoted from a non-canonical book, does that mean they regarded it as scripture?

Not necessarily. Christian authors throughout history quote from lots of different sources to help them deliver the message they intend. Every quote an author make isn’t necessarily preceded by the comment that what follows is or isn’t considered part of Scripture. It is taken for granted that the reader also knows what is and what isn’t Scripture. We can see this from a modern example:

In his book When I Don’t Desire God,¹ the Christian author John Piper opens each of his chapters with a series of quotes relevant to the topic of the chapter. Chapter 3, for example, opens with the following series:

Always you renounce a lesser good for a greater; the opposite is what sin is. . . . The struggle to submit . . . is not a struggle to submit but a struggle to accept and with passion. I mean, possibly, with joy. Picture me with my ground teeth stalking joy — fully armed too as it’s a highly dangerous quest.
(Flannery O’Connor, The Habit of Being)

Not that we lord it over your faith, but we work with you for your joy. (2Cor.1:24)

If we didn’t know more about John Piper (that he’s an evangelical Christian), we might mistakenly think that he regards both of the quotes above on the same par. It’s taken for granted that John Piper regards the Bible as God’s message to humanity, and that he’s simply quoting Flannery O’Connor because it’s a useful quote to help prepare the reader for what’s about to follow in the chapter.

Of course, if a writer specifically states that they regard a writing as Scripture then it’s clear what their opinion is.² But for someone to quote something, even highly favourably, doesn’t necessarily mean that they consider it scripture — an author could simply be using the quote because it’s a useful opinion that helps present their message.

Notes

1. Wheaton: Crossway, 2008
2. There are some examples of Christian authors referring to non-canonical works as scripture: e.g., Irenaeus (c. 130 — c. 202 AD) regarded a book called the Shepherd of Hermas as scripture (Against Heresies 4.20.2, cp. Shepherd of Hermas 26:1 (First Commandment)). There are also examples of Christian authors quoting a writing as containing something potentially useful, but not regarding it as scripture: e.g., Origen talks about a tradition some people use from the Gospel of Peter that helps them form their opinion on a certain topic (Commentary on the Gospel of Matthew, 10.17). And there are examples of Christian authors stating that certain works are rejected: e.g.,  Eusebius (b. 260) says the Gospel of Thomas was a rejected book (Ecclesiastical History 3.25.1-7). Other examples are less clear but, as we’ve seen in the main answer above, those ambiguous quotes don’t necessarily imply acceptance as scripture.