SSDs store in large blocks, say 64k or 2m. The seek to get to such a block is constant if it's not in cache; the read time is constant if not in cache; so 'SSD seeks are free' is basically correct. However, sequential IO is likely to land on the same block, in cache, in which case the seek time is essentially 0.

In practice, on hardware I have tested on, HDD's have a 10x or more slowdown from random IO relative to sequential, while SSD's have a 3x slowdown. Your hardware is definitely not mine, take this with a Hummer-sized grain of salt.