They aren't really third party apps in a samsung device. They are often much more integrated than the usual, google developed apps. Though I still disagree with the GP, because even providing more apps shouldn't lead to a 60gb OS size.
The problem is that they aren't first party apps. Yes, they are made by the same company but they aren't inherently part of the system. It's like if you called MS office a "first party app" and use this argument in comparison to Google docs. Just because Microsoft made both office and windows, doesn't mean that office is somehow more "pure" then using Google docs. It's all preference.
1st party doesn't mean "part of the system", it means built by the same people who make the OS. I said nothing about purity. I'd argue people's preference for "stock" Android have nothing to do with purity; they just have a preference for the apps google makes for Android because they like the experience better. Android has always had the capability to switch your default app, but there is a subset of people who prefer the stock experience. I don't know why that has to be some sort of cult like obedience rather than a rational choice
But Pixel is the 1st party google experience, and that's what those people are looking for. I'd hazard a guess they don't care about what default applications are in AOSP.
Then if you say "I'm looking for 1st party google experience" that's all good. But when you equate AOSP experience with Google experience, or what's worse: criticize other vendors purely on basis of them not inherently being google - that's when the argument stops making any sense.
(And looking in this thread, many people do exactly that)