Sorry, but I think your reading of that "FMC" rule is optimistically mistaken
Besides, it is unlikely that the ticket you lilnked to above would be issued on Air Serbia ticket stock - it seems far more likely that it would be issued on Air Canada stock. Air Canada only allows 1 23kg bag in Economy Class for free on transatlantic economy tickets.
Air Serbia has different baggage rules for European flights (BEG-LHR only qualifies for 1 23kg bag for free) than for transatlantics (2 23kg bags for free) - so to read all this as saying that you would beenfit from Air Serbia's transatlantic 2*23kg allowance when your ticket instead has a flight on Air Serbia that, by itself, only qualifies for 1*23kg - this being the applicable "First Marketing Carrier allowance" - and connecting to an Air Canada flight that, by itself, only qualifies for 1*23kg, is to read all these texts in a very hopeful manner.
But still, please let us know what you book and whether you can get a second bag for free [both as regards what the e-ticket says about the baggage allowance, and what the actual experience of travel is]!
(Having read the link you gave to tripadvisor, the "FMC" rule states that the baggage allowance of the flight of the first marketing carrier applies. In your JU BEG-LHR example, the baggage allowance is only 1*23kg bag, so despite what you apparently were told by the JU staff, Air Canada would not accept a second 23kg bag for free [and I suspect also that JU would not either]. See also the example I have reproduced below, where the "FMC" allowance of 0kg (on a domestic US segment) carries through and over-rides the free luggage allowance that would otherwise have been given on international JL-operated legs
(which is 2 bags).)
Your linked page itself links to
http://www.iflybags.com/
Try entering BEG to YYZ on JU, and compare the baggage allowance (1 23kg bag) versus BEG to JFK on JU (2 23kg bags).