He's grandstanding, for example "limited error checking." I have written commercial compilers for a computer manufacturer and a number of translators, DSL compilers, and an application generation framework. I wrote a Pascal to C translator in a week, but it took another month's work before it could translate any Pascal program, not just MY programs. The reason it took only a week is that Pascal's data structures and types map easily to C. Most importantly, I wanted to get rid of Pascal's variety of strict typing. It is brain damaged. By using C, I let the C compiler do the heavy lifting like code optimization. If I had been trying to map Python to Java, it would have been way more difficult, because of the typing mismatch. Python has dynamic typing and Java has static typing. Python to Java is so difficult that I doubt any sane person would attempt it.

Yes, you can write a compiler in a few days, if you have done it before, if your target language semantics are compatible, if you wave you hands about usability, if you are the only user, and you don't give a rat's ass about performance. If you want to translate to a low level virtual machine like JVM or to machine binary, you have a lot more work to do: compiler optimization, instruction scheduling, register scheduling, etc.

Tratt's three steps are something that you might find in a Wikipedia article (but I haven't looked.) Generally you don't create a parse tree, unless are doing something like mapping back into a newer version of the target language or doing sophisticated macro expansion. Using Bison or a similar parser generator, you directly create an Abstract Syntax Tree (AST). Using the AST, you perform semantic checking (has the variable been declared, type compatibility in expressions...); language based optimizations like removing redundant expressions, loop hoisting, tail recursion; generate abstract machine code. Using the AMC, do machine specific optimizations like instruction scheduling (reordering to take advantage of the architecture); register scheduling; and finally generate the binary. All of which take more than a couple of days