You are correct in this, but even moreso he didn't want competition from Glock, Browning (think Hi-Power) and a couple other importers that were shipping their guns to the US with higher capacity magazines. He was scared that his newer auto would not flourish and that would create problems selling his autos. His mindset was that he figured to keep the higher capacity magazines out of the US publics' hands would be better for his business. At that point in time many gun owners were on the only American made products kick, so he wanted level playing ground.
Bill also has stated many times how he just never saw the use in higher capacity magazines for pistols. He was not an advocate for hand guns, or carry guns at all and I think the issue was he recognized that his business started on revolvers and long guns so he wasn't familiar with the changing ways. Maybe it scared him to think he was going to lose a lot of market share, which he did anyway.
Ruger never really got better until a couple of years ago when the forward thinking minds realized what they needed to do to "be better". Now you see the LCR, LCP, mini-14 variants and more that probably would have caused Bill an aneurysm with his closed minded belief system.