so I think they're in what I thought was quite a small group of people who bought BDSP with no desire to buy PLA
My understanding is that such group is very much a small group and is, for the most part, a very small minority. I don't have hard data on this, of course, but I am fairly confident that 99% of the people who bought BDSP, if they did at all, probably bought PLA as well. Both games also happened to appeal to Sinnoh nostalgics (myself included in that regard) and I am fairly positive anyone who falls into the latter group bought both anyway, assuming they bought BDSP in the first place, that is. And I'm also confident the new kids who started Pokémon with Sword and Shield also bought both BDSP and PLA as well, or at least had their parents buy them both. There's a reason both games were marketed together from the start, after all.
If anything, it is much more likely based on my experience and what I've seen from people is that there are many more people out there who bought PLA but not BDSP. PLA on its own had a larger amount of appeal as a game to a bigger audience than BDSP because it's very different from past mainline Pokémon games in many ways and that in itself instantly made it significantly more appealing to both newcomers to Sinnoh and older players alike as a new and fresh experience. There are many people I've seen who didn't even consider buying BDSP because it was so faithful to the originals that there is significantly less incentive to buy it if you already played the original DPP unless you're looking for a simple nostalgia trip. Which is of course a totally understandable sentiment. The reverse case (people who bought BDSP but not PLA) is a much smaller minority, while the case of people who bought PLA but not BDSP is much larger because BDSP's target audience is smaller and more casual compared to PLA's target audience, making it a more likely scenario to know a person who only bought PLA and didn't touch BDSP at all.