We study the gas kinematics of a sample of six isolated gas-rich low surfacebrightness galaxies, of the class called ultra-diffuse galaxies (UDGs). Thesegalaxies have recently been shown to be outliers from the baryonic Tully-Fisherrelation (BTFR), as they rotate much slower than expected given their baryonicmass, and to have baryon fractions similar to the cosmological mean. By meansof a 3D kinematic modelling fitting technique, we show that the HI in our UDGsis distributed in "thin" regularly rotating discs and we determine theirrotation velocity and gas velocity dispersion. We revisit the BTFR addinggalaxies from other studies. We find a previously unknown trend between thedeviation from the BTFR and the disc scale length valid for dwarf galaxies withcircular speeds < 45 km/s, with our UDGs being at the extreme end. Based on ourfindings, we suggest that the high baryon fractions of our UDGs may originatedue to the fact that they have experienced weak stellar feedback, likely due totheir low star formation rate surface densities, and as a result they did noteject significant amounts of gas out of their discs. At the same time, we findindications that our UDGs may have higher-than-average stellar specific angularmomentum, which can explain their large optical scale lengths.