Sorry, confused you with @1design -- was about to leave and somewhat in a hurry... Apologies to you both
Apology accepted, it happens to the best of us
As you mentioned, it was surprising to see that Siglent decided to use EOL, and by now, obsolete parts in their design.
There are many ways to skin a cat when it comes to distributed filter design, what we see hare is blatant copying. No engineering went into that.
They did spin their own couplers and a few discrete filters, including the 10.7MHz IF section.
They did so little engineering that they even copied the 20dB fixed att + HMC307. There are better ways of doing it today, if they redesigned that, they would have been able to design it in a different way with modern components.
To me, it is more sad than anything else, the N9340B works, but it is far from being cutting edge design. It was a bottom of the barrel unit when released, it should have not been the reference for a design released 10+ years later. This is not a PXA level instrument.