A foreground mass can move a black hole's apparent weight at all because detectors never weigh anything directly. They record a waveform's amplitude and how its frequency climbs, and the masses are inferred from that pattern [16]. Amplitude is read as distance, and the inferred distance sets the redshift that turns observed frequencies into an intrinsic mass, so a wave arriving brighter than it left looks nearer, and therefore heavier, than it is [9][14].
Loudness on its own would be a weak argument, since a genuinely massive merger nearby is loud too. What the Max Planck group leaned on is shape: they matched the distortion pattern predicted by their lens model against the one in the data [6], and the paper describes GW231123 as both magnified and diffracted [5]. They report less than a 1% chance the pattern arose by coincidence [7]. That figure is about the data, not about the sky. It does not supply the prior odds that a compact object happens to sit close enough to the line of sight to do this, and with a sample of one event, that prior is doing quiet work in how convincing the result feels.
The arithmetic of the downgrade is worth seeing plainly. The two mass ranges do not overlap: the top of the lensed estimate, 180 solar masses, falls below the bottom of the unlensed one, 190 [11]. End to end, the lens takes 90 solar masses off the low edge and 85 off the high edge, cuts of 47% and 32% [12], and the midpoint falls from about 228 to 140 [13].
The part that outlives this one signal is directional. Magnification makes a wave stronger than it was at the source [9], and stronger waves are the ones a network finds, so lensed events should be over-represented precisely among the apparently most extreme mergers [15]. That is the same subset now cited as evidence that stellar collapse yields black holes heavier than theory allows [3], and Goyal and colleagues argue other exceptionally massive mergers may be distorted the same way and deserve a second pass as the detectors sharpen [10].
The thing this does not tell you is whether the lens is actually there. The model requires a compact object, plausibly an intermediate-mass black hole, embedded in a galaxy-scale field within a small angle of the line of sight [6], which trades one unconfirmed population for another. Nor does the lighter reading make GW231123 ordinary: at 100 to 180 solar masses combined, the analysis eases the tension with formation models rather than removing it [8].
