That set (and moreso the 8 piece set) ... real motley collection of steels and styles at first glance, but it comes together when looking at the actual needs of an enthusiast home cook:
-SK5 (an inexpensive carbon that still takes on some hardness, monosteel it seems) for a yanagi that will need to be very sharpenable and sharp but won't need endurance or durability since it will probably be treated with respect exactly because of missing routine. Reactiveness likely can be disregarded since that will be fish-only - and an all-hardened monosteel won't be as susceptible to deleterious rust as soft cladding.
-Shirogami (can be very sharp and thin, and is tough, and will have good edge retention in home use) for both the frequent-use santoku and the knife people will be using for the more infrequent but rougher jobs - the Ai-deba shape looks like a good multi-special-purpose deba/mincer/single bevel chef knife...
-Now SUS420J2 for the mukimono is an odd choice, you'd expect it to be too soft for something that usuba-like ... but hey, it is very nonreactive (fruit!) and might actually get darn sharp if the HT takes advantage of that steel just not having much in it that could get in the way of a fine edge...
The 8 piece set probably recognizes that the petty gets a lot of use with home cooks, so good choice for adding something finer looking without driving the price all too high...