The web page to which the first link you have given leads correctly quotes the notification requirement stated in Section 37 of the Immigration Act.
The web page to which the second link you have given leads correctly quotes
a) Section 84, under which an immigration office can issue a fine for contravention of Section 37(5), and
b) Section 76, under which a judge can impose a penalty for contravention of Section 37(5) if the foreigner is arrested and prosecuted.
Neither of these two sections states that "If a foreigner who did not make the notification of staying over 90 days is arrested, he will be fined 5,000.- Baht"