I would admit that last Saturday, the day after the product release, I could go into my local Best Buy and pick one up. I had expected them to be in short supply since that is how it typically is with Apple product releases.
Logically though, a $349 intelligent speaker competing at more than 3 times the price of alternatives far better established in the market, it is hardly surprising that sales volume might be low, or that sales could be slow to take off.
I could believe the supposition that Apple had them stockpiled as a result of manufacturing volume, and thus even brisk sales would have left plenty of stock in the retail channel, except that my Best Buy had not received many before launch, and had not had a stock replenishment, but still had 2 on the shelf, and more in the stock room when I bought mine.
I doubt Apple expected high volume sales. I think their expectation is to leverage sales amongst a large proportion of Apple Music subscribers, and for the HomePod to also leverage more Apple Music subscriptions. They aren't trying to compete with Amazon or Google so much as establishing a long-term strategy based on their own market of products and services. They know they have a great product that will sell as potential users get to hear them in action.