The word "מיומר" goes back to the translation of Brittish legislation during the period of Brittish Mandate over Palestine (the legislation was then published in English, Hebrew and Arabic). Some of the terms introduced by such translation were quite odd, as in this case. Anyway, the word "מיומר" was always (as far as I know) used to tranlate the word "purported" or "purporting to be".

Some examples from the legislation still valid in Israel:
Par. 17(2)(III) of Trading with the Enemy Ordinance (פקודת המסחר עם האויב, 1939):
"...decision, authority, direction or order given ... in pursuance or purported pursuance of the powers and duties conferred upon such custodian...", translated as:
"הכרעה, הרשאה, הוראה או צו שניתנו ... בהמשך, או בהמשך מיומר, לסמכויות ולתפקידים שהוענקו לאותו אפוטרופוס..."

Par. 136(1) of the Defence (Emergency) Regulations (תקנות ההגנה (שעת חירום), 1945):
"...any other information being, or purported to be, of military value...", translated as
"כל ידיעות אחרות שנודע להן, או מיומר בהן שנודע להן, ערך צבאי".

By the way, the phrase "purported invention" seems to be common in English usage:

http://www.lawyerscollective.org/^amtc/^Patent_Oppositions/p...
"The purported invention of methanesulfonic acid ... is not a new product and that it has been published previously. ...
We submit that the purported invention so far as claimed in the complete specification has been published before the priority date of the claim in a specification".

http://www.dww.com/newsletter/oct27_04.htm
"...it must be determined what the invention is and whether, in law, there is more than one invention. Once that determination is made, the four-step test for obviousness (the Windsurfing test) is applied to each purported invention".