That link is dead but I found it here:
On page 8, Rheem recommends a flexible water connection "such as PEX" for both the hot and cold connections. The diagram shows the flexible connection extending right to the connection nipples. This is the recommended (preferred) installation.
Also, Code Enforcement of the North Carolina Plumbing Code has a Q&A for plumbing questions:
On page 16 it says:
Question: Could you put me in the right spot in the code book where it states that 18” of copper is required on a gas water heater? We have been questioned about PEX being tied directly to an electric heater, which we know is legal, just trying to locate the verbiage in the code book.
Answer: It is not in the code. Almost all manufacturers require a minimum of 12 inches of clearance above the top of the water heater to combustibles. Copper extensions have always been used to meet this requirement. So many manufacturers have that requirement it is assumed it is a code requirement. On electric water heaters, we go by what the installation instructions require. Most have a zero clearance above the top of the heater and do not prohibit connecting PEX or CPVC directly to the tank, but there are a few we have found that does. It will go back to the installation instructions.
Also, the maker of Zurn PEX has an installation guide here:
On page 26 it says this:
Connecting Zurn PEX to a Water Heater Gas Water Heaters Zurn PEX must be kept at least 6" away from the exhaust vent of a gas-fired water heater. This is easily accomplished by using flexible water heater connectors. When the inlet and outlet connections are well away from the exhaust vent, such as most heaters with side connections, Zurn PEX may be directly connected to the water heater. When connecting to a power vented gas water heater, Zurn PEX tubing should be kept at least 6" away from a metal blower housing. If the blower housing is plastic, the Zurn PEX tube may be directly connected to the water heater. Electric Water Heaters Zurn PEX may be connected directly to electric water heaters for residential plumbing applications. Zurn PEX has brass male and female threaded adapters and swivel adapters that can be used for this application. Tankless Water Heaters Because of the extremely high temperatures produced by tankless water heaters, most manufacturer’s require metal piping be connected to the heater’s supply and discharge. When installing Zurn PEX near a gas fired tankless heater, keep Zurn PEX tubing at least 12" away from exhaust piping, which can reach temperatures of 460°F
Basically, if the water heater manufacturer doesn't prohibit a direct PEX connection, it's fine. It does appear to make good sense to keep PEX away from the heat of the exhaust vent of gas fired appliances but electric tank water heaters don't have that issue. In fact, a direct PEX connection is the preferred connection by some manufacturers.