That was my thinking all along. Then a comment from
@unfocused impelled my to look up that the horizontal pixel count of the R5 is exactly that needed for 8K DCI video. The implication is that the R5 is 45 MP, as opposed to something higher like 50 MP, because of video.
I concur... Canon made a statement for first 8k video in a hybrid body with the R5 driven by the DCI standard... similar to the 5Dii video and letting their engineers out the chain vs magic lantern/5Diii etc.
For 8K video, the devil is in the details with the A1. No raw (even with crop) ie only via codecs with 200/400Mbs to V60/V90 UHS-ii cards. They use the full 50mp sensor width and oversample to 8K but only 4:2:0 rather than 4:2:2. The A1 records 8K UHD only vs the R5 DCI or UHD standards. The recording time limits on the A1 are based on higher internal temperatures which Canon added via firmware option a long afterwards initial launch.
=> this hasn't stopped Sony advertising 8K/30 for the A1 but it isn't as well rounded as the R5. The market hasn't turned against them for its limitations.
From DPR... acknowledging the sensor resolution decision by Canon for 4K
"The a1 has an oversampled 4K mode of its own, coming from an APS-C crop of the image; however, it comes from 5.8K capture, and so can't quite match the
perfectly oversampled 8K-to-4K capture of the Canon. Both models have a 4K/120p mode, and the Sony takes an edge with greater detail though it does require a 1.13x crop while the Canon requires none."
I believe that the R5/R6 surprised Sony/Nikon etc (and their fanbois) as Canon was late to the party for mirrorless and hadn't brought out a sensor with a step (vs incremental) technological change.
Canon = R5 will have 8K => Market = timelapse
Canon = R5 will have 8K video => Market = maybe 15 second due to overheating
Canon = R5 will have full 8K video => Market = maybe but no IBIS/AF
Canon = R5 will have full 8K video with IBIS/AF => Market = not possible with overheating etc otherwise Sony etc would have already done it.
=> Hence the overheating storm in a teacup. The simple firmware change to actually measure the temperature vs timers made a big difference.