I think you need better reading comprehension or at least the basic understanding of how semiconductors work.
Please explain it better for me.
Since we do not know power levels in the leak, they could be passively cooled, air cooled, water cooled, liquid nitrogen cooled, etc. It could be at 15W or 320W or someplace in between. A single core in 1T could easily consume 20W, 30W, or more. For 1T, that wide variety of power levels and wide variety of possible cooling gives us a wide range of possible frequencies. Way more range of frequencies than would give the +10% 1T performance that we are discussing. We are talking easily a 4X range of frequencies, if not 5X depending on how the test was performed. So, without knowing any more information of how it was run, a 4X range of frequencies to get a +10% performance increase gives us no real estimate of IPC.
If we knew the test was iso-power then you'd have a point. But we don't.
But for MT, the possible range of frequencies is drastically smaller. This is because the range of power that can be given to each core is much tighter. You can't say give 20 W or 30 W of power to each cores because that would blow the power budget. Heck, you couldn't give 7 W each* in 320 W total PL2. One could pretty accurately estimate the power (plus or minus a bit), the frequency, and thus the IPC.
* Note: P cores would have different power levels than E cores, but separating them out in this paragraph made it too complex to explain to you.