Interesting report. The reference paper link indicates the SMBH is about 30 million solar masses, and states "We note, however, that the appearance of such narrow X-ray re-emergence peaks requires the duration of the flare that illuminates the disk to be shorter than the timescale of the total response from the disk. The total duration of the flare is 10 ks. The model requires that the disk only respond to the first part of the flare, before 4,500 s, for the narrow peaks to appear at 6,800 s without being smeared out." Article Light bending and X-ray echoes from behind a supermassive black hole
The space.com report indicates the BH is about 800 million light years distance. 30 million SMBH diameter is about 1.185 au size. Light time across the diameter of the SMBH is about 591 seconds. The SMBH diameter to resolve using a telescope, angular resolution about 4.83 x 10^-6 mas needed

5000 seconds light time is about 10 au size or diameter.