Sure, but you said single threaded. Those extra cores don't matter. They're not under load, therefore they produce negligible heat.
At the end of the day, these are still 14nm parts. A single 10900K core can be considered "pretty much" the same as a single 6700K core. They have the same architecture and IPC. At 4GHz, both parts will perform identically.
That means that with one core, you're dealing with "pretty much" the same amount of heat, over the same amount of area, at the same clockspeeds and voltages. Add a core, you double it, add a core, you triple it, add a core, you quadruple it. You've now built a 6700K. Now add 6 more of those cores, you've built a 10900K.
Now granted, a 10900K core is going to do this at lower voltage and with less heat, because of the refinements of the manufacturing process, but that only works in the favour of the later chip.
If you load 4 of those more efficient, later production 10900K cores, you'll get a reasonable amount less than 6700K heat. If you load all 10 cores you get 10900K heat. If you load one single core then you'll get substantially less heat than either of those circumstances, which means cooling a single threaded workload is simply not an issue - if you're only pursuing the same clocks, anyway. Intel always tries to use as much of the available headroom as possible, which is why the single core boost always goes up, from 6700K to 7700K, 8700K, 9900K, and now 10900K. They're not really producing more heat when in single threaded workloads. They're just producing lots more in multi-threaded workloads.
Single core boost will always go up as long as manufacturing keeps improving. The battle is in maintaining high all-core boost clocks as you add more and more cores into the same space.