Is one transit enough to be confident that we detected an exoplanet?
I will argue:
and back it up with material contained in my earlier question After only one eclipse of its X-ray bright primary, how can astronomers estimate the first extragalactic exoplanet's period to be about 70 years?
Phys.org's Astronomers may have discovered the first planet outside of our galaxy links to Di Stefano et al. 2021 A possible planet candidate in an external galaxy detected through X-ray transit (earlier arXiv preprint) and says:
While this is a tantalizing study, more data would be needed to verify the interpretation as an extragalactic exoplanet. One challenge is that the planet candidate's large orbit means it would not cross in front of its binary partner again for about 70 years, thwarting any attempts for a confirming observation for decades.
"Unfortunately to confirm that we're seeing a planet we would likely have to wait decades to see another transit," said co-author Nia Imara of the University of California at Santa Cruz. "And because of the uncertainties about how long it takes to orbit, we wouldn't know exactly when to look."
Can the dimming have been caused by a cloud of gas and dust passing in front of the X-ray source? The researchers consider this to be an unlikely explanation, as the characteristics of the event observed in M51-ULS-1 are not consistent with the passage of such a cloud. The model of a planet candidate is, however, consistent with the data.
See the excellent answers there to get a better idea of the great extent to which the authors have build a scientific case for "exoplanetship" without actually calling it an exoplanet.
Unless otherwise stated, orbits of planets (exos or in our own system) are pretty much periodic and I think that closed, well-defined and roughly periodic orbits are part of the definition of what a planet is, even if it's not explicitly stated as such.
How do we know if a transit is from an orbiting body?
As you point out transit periodicity is a hallmark of an exoplanet identification using the transit method (as it is for spectroscopic methods) and without a few repeats you don't have periodicity.
Accretion material, starspots, and unkown/unidentified intervening bodies are just some of the ways that a single dip might happen.
So even with all the circumstantial evidence cited within Di Stefano et al. 2021, the authors are careful to make clear the difference between a dip consistent with that of an exoplanet, and a likely exoplanet or confirmed exoplanet.