Agree. The weight isn't super difficult even with a steel fork. Not easy, just not ridiculous. You kind have to be a little, but not world-class, weigh weenie-ish for all the parts including light tires and tubes. An ounce or two here and there adds up over an entire build if you're looking for a goal weight. Even look at skewers, rim tape and bar tape - again, see if you can save a bit for what you'd spend anyway.

The large tire size, not so much. Large tires and tubes and the wheels to accommodate them would add a very significant amount of weight to the build, compared to reasonably light alloy wheels and lightweight 23-25mm tires and lightweight tubes. If you're going modern-retro with downtube indexed shifters, that helps quite a bit compared to integrated brake shifters.

As far as the frame and fork, my limited experience is that a lot of vintage nice high end or near-high end framesets (i.e. double butted 531 or SL grade steel) will allow you to get in that 20 pound range without being too awfully crazy about it. Just a little bit crazy, looking to save an ounce or two every time you pick a component. This stuff is fun and you can do it with some recreational time on eBay or finding websites that sell out stock.