Did you remember to hit Apply after removing those marks?
I think I did. I try again when I get home today from work.
Ok, I tried again removing marks of those other cores, clicked apply and then ran again s-tui. This time it seems to have effect. lscpu | grep MHz gives now this:
[mardix@mardix-81x2 ~]$ lscpu | grep MHz
CPU MHz: 3365.538
CPU max MHz: 2700,0000
CPU min MHz: 1400,0000
But if I run inxi -Cay I get this:
[mardix@mardix-81x2 ~]$ inxi -Cay
CPU:
Info: Quad Core model: AMD Ryzen 3 4300U with Radeon Graphics bits: 64
type: MCP arch: Zen 2 family: 17 (23) model-id: 60 (96) stepping: 1
microcode: 8600102 L2 cache: 2 MiB
flags: avx avx2 lm nx pae sse sse2 sse3 sse4_1 sse4_2 sse4a ssse3 svm
bogomips: 21570
Speed: 1397 MHz min/max: 1400/2700 MHz boost: enabled Core speeds (MHz):
1: 1397 2: 1625 3: 1723 4: 2960
Vulnerabilities: Type: itlb_multihit status: Not affected
Type: l1tf status: Not affected
Type: mds status: Not affected
Type: meltdown status: Not affected
Type: spec_store_bypass
mitigation: Speculative Store Bypass disabled via prctl and seccomp
Type: spectre_v1
mitigation: usercopy/swapgs barriers and __user pointer sanitization
Type: spectre_v2 mitigation: Full AMD retpoline, IBPB: conditional, IBRS_FW,
STIBP: disabled, RSB filling
Type: srbds status: Not affected
Type: tsx_async_abort status: Not affected
[mardix@mardix-81x2 ~]$ cpupower frequency-info
analyzing CPU 0:
driver: acpi-cpufreq
CPUs which run at the same hardware frequency: 0
CPUs which need to have their frequency coordinated by software: 0
maximum transition latency: Cannot determine or is not supported.
hardware limits: 1.40 GHz - 2.70 GHz
available frequency steps: 2.70 GHz, 1.70 GHz, 1.40 GHz
available cpufreq governors: conservative ondemand userspace powersave performance schedutil
current policy: frequency should be within 1.40 GHz and 2.70 GHz.
The governor "performance" may decide which speed to use
within this range.
current CPU frequency: Unable to call hardware
current CPU frequency: 2.83 GHz (asserted by call to kernel)
boost state support:
Supported: yes
Active: no
When I run auto-cpufreq --monitor I get this:
Oh well, I guess I have to believe turbo boost is on then… 
Run it with sudo.
Yeah, we had some old threads here about the tools like inxi not reporting the right frequency of AMD cpus. I wouldn’t worry too much about it. If at least one of the tools managed to get a high frequency read, I’d say you’re golden.
Yeah, one would be my thread! And I got a “mild” RTFM admonition by the developer of the tool.
(link further above)
Thanks all of you who helped me with this! This thread taught me a lot of new things about my laptop. I guess I can mark this problem solved now.