I think their remark wasn't directly aimed at you, but at an imaginary publisher. It could have been worded better (as it sounds a bit offensive, indeed), that is sure, but if my interpretation is correct I believe they have a point.
I have heard many times, especially from GOG users, people saying that they bought the same game twice, first form Steam and then from the "delayed" GOG release. That surely encourages a certain way of thinking: first of all, in the beginning developers will put their products exclusively on Valve's store (helping in reinforcing their already achieved victory against any form of competition -I consider every store offering Steam keys as Steam itself) for convenience, since they know that the majority will abide to any kind of rule they impose; then, maybe, they will put them on other stores knowing that some people, especially those particularly caring for the DRM-free concept but not enough to "boycott" DRM'd games altogether, might even buy the game again -a sick strategy, to say the least. That way, once they have sold all they could on the main platform, they can sill get more sales from them and from the last few people who neither bought the game nor have resorted to piracy. (An impression Deep Silver in particular gave me. After the games have been sold at 80% discount countless times and bundled to oblivion as Seam keys they appeared here, but the newer ones, even if DRM free -only in Germany, but they have multiple language packs included anyway- have yet to show up. I don't believe in coincidences.)
Imo, to support the developers and the DRM-free concept would be indeed better to buy twice in the same place, or better, more than one DRM-free copy, regardless of where; buying first on Steam and then anywhere else contributes to keep those who desire no DRM second class citizens living of discards. See the big publishers on GOG: regardless of what anyone may think, they distribute here only what they consider leftovers -and why shouldn't they, since the conditions of their sales on Steam follow their own rules, respect for the customer being only incidental if considered at all, basically granting them a free and constant market analysis and retroactive control on the product?

I always say that I find absurd how economics works in the videogame industry the inverted way as everywhere else: here, the customer base must adapt to the producers' demands (otherwise Gaben's behemoth will just crush them and they can forget about playing legitimately bought videogames) rather than influencing the market.
If people stopped accepting this, they could only benefit. Everyone minus Valve could only benefit, actually, and I think that many major publishers are starting to understand this as well: problem is, they want to “rebel” by pushing their own Steam clones rather than letting users breathe.