No, he is not joking. Unix is designed to work on many types of text terminals. That means it has to work correctly in an environment where bold, underline, italics, reverse, and colors are not available. This implies that man pages have to be written without relying on such types of text rendering.
Note what it says in the Debian man page you quoted above: "Exact rendering may vary depending on the output device. For instance, man will usually not be able to render italics when running in a terminal, and will typically use underlined or coloured text instead." Well, that statement is technically correct, but very optimistic: on many types of terminals, neither underlined nor coloured text is available, nor bold.
Now seriously, given that today real hardware terminals have become nearly non-existing (we had a discussion about that here recently) and instead everyone uses terminal emulators, most of which run on GUIs or on VGA-style consoles. These days, the ANSI (a.k.a. VT[12]xx) command set is the de-facto standard for text-mode terminals, and one can safely assume that all terminals support bold, reverse, underline, and colors. The ANSI commands for italics exist, but are rarely supported. One could make a deliberate design choice, for example in FreeBSD (there is no longer an entity capable of controlling all Unix, that market is now fragmented) that all terminals must have those capabilities, and then change all programs that output text to use those capabilities. Such a decision would require a heck of a lot of discussion, and would take accessibility into account: color blindness, whether italics looks "different enough" form regular, and so on.
It seems that Debian has made that decision, at least for man pages. Well, good for them. I wonder how consistent they have been; for example: Are error messages in compiler output now underlined bold, and warnings in italics? Probably not. Another reason for me to not use that operating system.
And if you want to claim that "everyone" uses Unix machines with a GUI, that's just nonsense. As an example, I use somewhere between a dozen and two dozen Unix machines regularly (about 1/3 personally, the rest for work), and only one of them has a GUI attached; all others are accessed via text mode (ssh sessions).