If seek times are at the heart of it, then seek times are. Seek times can be high, even with high IOPS, though, and low with low IOPS.
It hasn't proven anything of yours correct. IOPS are one measure of throughput, just like MB/s is. For a given workload, MB/s is equivalent to some IOPS rating, and we use that.
IOPS are just another measure of throughput. A formula to show exactly that has been put forth in this thread multiple times. Sequential transfers are not necessary for it to be true, simply transfers of agreed-upon size and layout (IE, 64KB, random access).
Also, you've now changed the goalposts. That SSDs can sustain more IOPS than HDDs was not the matter in question, but the statement that IOPS and throughput were not equivalent measures of performance. Nobody argues that HDDs can sustain more IOPS than SSDs, nor has argued that.
For example, if IOPS and throughput are different, should we start considering CrystalDiskMark screenshots useless, as diagnostic aids? After all, it gives results in a throughput measurement, not IOPS? Of course not. The throughput results tell the same story, just with a different dimensionality.