He was referring to toilets of the past. The old low boy one-piece toilets that came with high volume fill valves. Some of those used 9.5 gallons on the flush. We're talking 80's and older.
New toilets, since 1992 have used 1.6 gallons or less. It has been at least that long since the one-piece toilets are now like the two-piece toilet counterparts. You have a tank above the bowl, and the water drops through the flush valve and flushes the bowl.
Is your guy making stuff up? More like recalling ancient history month.
A current one-piece Ultramax toilet by TOTO
Kohler Santa Rosa, with flush canister and a Fluidmaster fill valve.