That's exactly correct. If you were coming from OD&D and transitioning to AD&D, you likely used the 3d6 as a default and were just adapting the new rules.
The four methods of rolling in the DMG (which is why the UA method is, of course, Method V) are listed as
alternatives to the assumed baseline of 3d6, in order.
After discussing how 3d6 can create marginal characters that might discourage new players, the DMG says "Four
alternatives are offered for player characters:" (emphasis mine).
To put it more bluntly- AD&D is a codification and expansion of the OD&D rules (despite whatever certain lawsuits might have alleged). 3d6, in order, is the default, and acknowledged as such. The methods proffered in the DMG are alternatives to the default.
Whether it's because it was listed first, or because it's by far the easiest to implement, 4d6k1 became the alternative that most people were familiar with. Perhaps because rolling twelve characters and selecting the one you want sound annoying (Method IV).