That's actually a standard set and is mentioned on the analysis. Gyro Ball is a generally better option, sure, but HP Ice lets you beat dragons and prevents Gliscor/Landorus-T setting up on you.Forretress with Hidden Power Ice and (thankfully) no SpA investment.
I'll admit it does have a tiny bit of merit in hitting Pokes with a 4x weakness (Dragons specifically... ScarfMence takes about 60%). However, 3HKOing Landy-T and doing 25% MAX to my Hippo aren't getting you anywhere.
Also, this is the minimum Landorus-T is taking:
0 SpA Forretress Hidden Power Ice vs. 252 HP / 0 SpD Landorus-T: 160-192 (41.88 - 50.26%) -- 44.14% chance to 2HKO after Stealth Rock
However, Naive is common on defensive Landorus-T and more offensive variants don't run this much bulk.