Q1. Why does a 100%K lead to a dull black?
Does not "lead" to dull black. Process black is a transparent ink. When you have a large area of transparent ink you will notice that transparency... a grayish black or dark gray.
Q2. Why would a solid black lead to offsetting?
Does not "lead" to offsetting. In any print process, there is always a little misalignment. You have a tolerance to offset, it depends on the quality of the machine, the skill of the operator and the overall quality of the printer's policies, but always exist.
There is also an intrinsic misalignment called a "rosette" where the 4 inks have different screen angles.
If you have a large mass of black, a tiny misalignment does not matter, but in a tiny font, yes, you notice that. If you have a thin small font, this little misalignment could be let us say the total width of an "I" perhaps?
One additional comment. Solid black is not the same as rich black.
Solid black is 100% K. It can be using Process black (the ink used in a cmyK print) or can be using another type of black using a spot color (deep black for example) at 100%.
Rich black is a combination of inks to darken the black, normally using a combination of them, and the actual black ink does not necessary need to be at 100%. There are many combinations for rich black. I normally use what the color profile indicates me, for example in the SWOP v2 specification, the darkest RGB black when converted to cmyk is c75m68y67k90.
In the case of the text, he is using the word "Solid" just as a metaphor, to the perception of "solid", he could have used the word deep, dark, etc.
Related: When should I use rich black?