Fair point, though those are not the ones that people were talking about since the 90s—but then nobody could imagine you could wrangle the railway (a national service with a peculiar culture) into cooperating with the metro (a municipal one), that’s unironically an achievement of the current city administration.
That these are being “built” is once again a bit too strong a statement—the rail lines were already there and in use for cargo and the occasional suburban train. (The veeery slooow speeds at which the latter went between the big stations had relegated them to the sole use of urban arcanists and the occasional exhausted hiker disembarking from his long-distance train.) There’s some renewal of the aging rails, to be fair, and the stations are new.
Two problems with these:
First, the stations are too few and once again too far away from anything else. (Nobody’s fault—the rails are sometimes a century older than the metro and the outward creep of the city limits.)
Second, unlike on the ring, the trains on the chordal lines (I refuse to call them “diameters”) are operated according to the railway rules, meaning there are too few of them to be able to disregard the schedule, the schedule itself is at best a suggestion (and woe is you if you don’t keep track of train cancellations), and the stupid multi-hour midday “maintenance breaks” are still in place.
I’ve used these lines for weekly commutes, and if they fit your problem well, they can be very useful. But overall I’d say they are even more situational than the ring, and it’s frustrating compared to my utopian headcanon of how well they could work. Or hell, to the RER lines in Paris, even if those connections are no joke either—nevermind the Barcelona Rodalies, which are (or were, two decades ago) the platonic ideal of suburban/urban rail transport interconnection.