I now can do a IBAN like: NL33INGB0123456789 and the extension will say its valid.

But its not valid because of this validation is missing:

An IBAN is validated by converting it into an integer and performing a basic mod-97 operation (as described in ISO 7064) on it. If the IBAN is valid, the remainder equals 1.^{[Note 1]} The algorithm of IBAN validation is as follows:^{[8]}

Check that the total IBAN length is correct as per the country. If not, the IBAN is invalid

Move the four initial characters to the end of the string

Replace each letter in the string with two digits, thereby expanding the string, where A = 10, B = 11, ..., Z = 35

Interpret the string as a decimal integer and compute the remainder of that number on division by 97