You are confusing two different statements.
The first about basic numbers which has a supposition that most of the Clan forces involved will be 5/8 or faster while the IS forces will be 4/6 to have comparable firepower- yes Quickdraws, Dervishes, Wolverines, Griffins and Shadow Hawks but they will not suffice to deal with Summoners & Timber Wolves. Most IS heavies of the Invasion were 4/6, many of the heavier mediums (ENF, CN9, HBK) are that speed and the faster assaults are also 4/6. So, 5/8 or faster Clan force against IS 4/6 force is going to get those numbers when able to run at a oblique as I described. I also included the option of walking, either forward or backwards. Fighting favors the Clan force that can stay out 16 hexes or more- I have 9 hexes of ERLL, 7 hexes of ERPPC, and 6 hexes of Gauss plus UAC/5 & LB-5X. The IS has 3 hexes of PPC & AC/5, 5 of LRM (same as Clan), and of course AC/2. With the imbalance in numbers, the damage potential being tossed back and force should be about equal but the Clan force will hit more often- for bigger hits.
The second part of the statement, which was why it was a separate paragraph, discussed the scenario as I played it out- which gave extremely favorable terrain to the IS. I described it but did not spell it out, so here- I played with roughly 8 BT Canyon maps, 6 rolling hills and I think it was 2 'Battletech' that had the hill & pond. LG's IS force had 6 jumping mechs though most were lighter while my Clan force had a single Pouncer able to jump though all fell in that 'or faster' category (Linebacker, Adder, Ice Ferret) so able to mostly keep favorable move mods (+3 when running, +2 when walking).