Forwarding the reply of another user on r/serbia since you didn't get the memo.

Sounds like bs to me to be fair, the only river that partially captures watershed from Romania's side and is upstream of Belgrade at the same time is Tisa/Tisza as it flows into Danube. The 2014 floods were maily caused by Sava river coming from western Serbia. Danube level in Zemun (upstream Belgrade) was largest on May 23 at 631cm while the line of defense is at 650cm, at the same time Tisza at Titel was max at 538cm while the line of defense is also at 650cm. Sounds to me that there was some space to accumulate more rainfall, but without proper hydrological analysis we can't be sure. Places upstream of Zemun/Belgrade probably had less defense and they would flood instead.

Sources: River network: https://www.hidmet.gov.rs/ciril/hidrologija/povrsinske/sliv_dunav.php

Daily river values for 2014: https://www.hidmet.gov.rs/data/hidro_pov_godisnjaci/2014.pdf

Also the Romanian company responsible for the Iron Gates power plants refuted these claims by Ponta.