Yeah, that is wrong. There are slaves out there that may only support 9999 or less, but that shouldn’t drive the design of a modern master or TCP implementation. It’s laughable that the site with a copyright date of 2017 calls it “Standard Modbus” and then ignores the current specification.
If the docs for a slave device are adding 40000 to the PDU address of a holding register, then that is bad documentation that leads to more confusion. So we have little idea what 40001 means until we test it because docs that do this usually don’t tell you how to convert their fairy tale number to a PDU address. So we guess, is it 0, 1 , 40000 or 40001? Complete madness.
For ntsib’s generator, is the PDU address 8193 or 8192? (I suspect it is 8193) For the client, do you use 8192, 8193, 8194, 48192, 48193, 48194? Simply Modbus. 
Okay, done ranting. Should we talk about endianess now?