Agree completely.
Add to that complexity the fact that the initial imagery is both raw data and electronic (CD image capture) and you add a lot of interference that needs to be filtered out.
Your point about many short exposures from separate observatories is probably a key factor in the discovery since, with a calculated 27,000 year orbital period, any reasonable observation period (expressed in fractions of a human lifetime) would, necessarily, encompass only a very short segment of a single orbit of the planet. Getting the planet to emerge from observational “noise” had to require massive computer processing followed by cross matching between the participating observatories to confirm they were really looking at the same distant point in space.
It really is a remarkable achievement.
(I assume that their initial interest in the star was it’s relatively young age (4.5 million years) and curiosity about what could be observed about early planetary formation within its accretion disc. The fact that there was already at least one discernible planet observable using the transit method probably came as a surprise.)
I am not sure how they computed the period of the distant planet, but I am nearly certain it was not from angular measurements. If they have the period of the near-in eclipsing companion planet and can measure the associated Doppler shift of its star, they provides constraints on the size of the small orbit and therefore mass of the star. Knowing mass of the star and distance to the planet one can estimate the period of a circular orbit.