If i have to rationalize, They could be experimenting with Oreshnik like capability,to target high value sites far away using multiple submunitions, with this concept of small ICBM.
Tho ukraine is doing that with cheaper cruise missiles.
Still, will be Intresting to see how it progresses.
Except that capability requires rapid launch on demand, which by extension necessitates solid fuel.
What do you think the RS-28 is? Or R-29RMU2?
The Sarmat is designed to fill a very R36-shaped hole in Russia's silo-based launch requirement, a holdover from the time they were competing with the Titan-II.
A solid-fueled missile is significantly better & easier to work with, as the US with their Minuteman (and upcoming Sentinel replacement) would tell you.
The Sineva is similarly a descendent of the liquid fueled R29 from the 70s. The Russians' problem is that they seem to think it's too expensive or risky to switch over to solid fuel entirely. This can be true especially when you've built out a significant knowledge base & infrastructure surrounding liquid propulsion like the Soviets/Russians have, it can be quite economical to just carry that forward.
But when you're building out a new capability for the first time like the Turks are apparently attempting, I can't seem to think of a good reason as to why you'd want to go with hypergolic fluids. Especially if they want it to be capable of tactical use cases.
The only explanation I can think of is that they originally started off trying to make a light/medium-class orbital rocket and are using the same baseline configuration for an ICBM (kinda like Titan-II's story).
I'll believe it when I see it though. A lot of people I'm talking to on Turkish defence forums seem to think this missile (and the agency behind it) are a bit of a sham, with no background or pedigree in defence R&D.