Here's the relevant
rule. You can see that the Invalid Signal rule refers to "any waving signal" with no need to be above the head or shoulder.
Valid Signal
ARTICLE 2.
A valid signal is a signal given by a player of Team B who has obviously signaled his intention by extending one hand only clearly above his head and waving that hand from side to side of his body more than once.
Invalid Signal
ARTICLE 3.
An invalid signal is any waving signal by a player of Team B:
- That does not meet the requirements of Article 2 (above); or
- That is given after a scrimmage kick is caught beyond the neutral zone, strikes the ground or touches another player beyond the neutral zone (A.R. 6-5-3-III-V); or
- That is given after a free kick is caught, strikes the ground or touches another player. [Exception: Rule 6-4-1-f]
Fair catch signals are also explicitly reviewable per Rule 12, article 4g (page 115
here):
Kicks
ARTICLE 4. Reviewable plays involving kicks include:
a. Touching of a kick.
b. Player beyond the neutral zone when kicking the ball.
c. Kicking team player advancing a ball after a potential muffed kick/fumble by the receiving team.
d. Scrimmage kick crossing the neutral zone.
e. Blocking by players of the kicking team before they are eligible to touch the ball on an on-side kick.
f. A player touching or recovering a kick or loose ball who is or has been out of bounds during the kick.
g. Receiving team advancing after a fair catch signal
The NFL rule does include the above the shoulder part, which might explain some of the confusion.