Jerry,
The signature is a generalization. Get it? Intended to be a joke. 
If I had stated "All generalizations are true", it wouldn't have been funny.
From Merriam-Webster online: (bolding and colors are theirs)
Main Entry:1joke Pronunciation: \ˈjōk\ Function:
noun Etymology:Latin
jocus; perhaps akin to Old High German
gehan to say, Sanskrit
yācati he asksDate:1670 1 a
: something said or done to provoke laughter;
especially : a brief oral narrative with a climactic humorous twist b (1)
: the humorous or ridiculous element in something (2)
: an instance of jesting
: kidding <can't take a
joke> c
: practical joke d
: laughingstock2
: something not to be taken seriously
: a trifling matter <consider his skiing a
joke — Harold Callender> —often used in negative constructions <it is no
joke to be lost in the desert>
Also from M-W online:
Main Entry:gen·er·al·ize Pronunciation: \ˈjen-rə-ˌlīz, ˈje-nə-\ Function:
verb Inflected Form(s):gen·er·al·ized; gen·er·al·iz·ingDate:circa 1751
transitive verb1
: to give a
general form to2 a
: to derive or induce (a
general conception or principle) from particulars b
: to draw a
general conclusion from3
: to give
general applicability to <
generalize a law>;
also : to make indefinite
intransitive verb1
: to form
generalizations;
also : to make vague or indefinite statements2
: to spread or extend throughout the body
— gen·er·al·iz·abil·i·ty \ˌjen-rə-ˌlī-zə-ˈbi-lə-tē, ˌje-nə-\
noun
— gen·er·al·iz·able \-ˌlī-zə-bəl\
adjective
— gen·er·al·iz·er
noun
And:
Main Entry:1false Pronunciation: \ˈfȯls\ Function:
adjective Inflected Form(s):fals·er; fals·estEtymology:Middle English
fals, faus, from Anglo-French & Latin; Anglo-French, from Latin
falsus, from past participle of
fallere to deceiveDate:12th century 1
: not genuine <
false documents> <
false teeth>2 a
: intentionally untrue <
false testimony> b
: adjusted or made so as to deceive <
false scales> <a trunk with a
false bottom> c
: intended or tending to mislead <a
false promise>3
: not true <
false concepts>4 a
: not faithful or loyal
: treacherous <a
false friend> b
: lacking naturalness or sincerity <
false sympathy>5 a
: not essential or permanent —used of parts of a structure that are temporary or supplemental b
: fitting over a main part to strengthen it, to protect it, or to disguise its appearance <a
false ceiling>6
: inaccurate in pitch <a
false note>7 a
: based on mistaken ideas <
false pride> b
: inconsistent with the facts <a
false position> <a
false sense of security>8
: threateningly sudden or deceptive <don't make any
false moves>
This would probably be known as "taking a joke too far".