Stamped/folded yes, but I don't think there is any welding. The breech block and corresponding hardware are secured in the slide by dual pins (actually, 4) . Said pins can be driven out, and the internals of the breech block can be accessed for detailed cleaning and maintenance. When reassembled, new pins are to be used.
I think the folded/pinned guns are a bit lighter, significant on a duty belt full of other gear. The .45 acp is a low pressure round, durability on a P220 will not be an issue. The milled/stainless P-series surfaced with the higher pressure .40 S&W round and eventually became standard for all P-series later in produciton (I think).