Yes but that would only mean that 3 to 5 times the volume would need to be added (to the retentate), and I suspect the volume would need to be 1000 times bigger to make the spiking straightforward. That's because Ashenden says "So if you wanted to add 0.0037674 UI you would add 0.000000113 ml (i.e., 0.6ml divided by 20,000/0.0037674 = 0.000000113 ml) of the synthetic EPO solution. This is 0.113 microlitres." But 0.000000113mL is 0.113 nano-litres (for the non-geeks that's 1000 times smaller, which really is tiny and it could well be difficult to spike accurately even with dilution techniques.) So maybe he is right in essence, and the source of the confusion is that he has just befuddled his units.
I also wondered about why he focused only on spiking the retentate. Surely one would need to spike the whole pot of p1ss, that the 20ml for filtering was taken from in the first place?....unless the whole pot is filtered, which would seem odd to me (especially as they asked for permission to split it and re-test after the results were out, so there must have been some p1ss left). If the idea is to run the test once and calculate the volume of rEPO required to spike the whole pot, the spiking volume should be big enough to be practical. But that calculation would depend on knowing what fraction of total isoforms survives the filtration process - do you have a view on whether this information might have been available? Regardless of whether it's doable or not, running tests on samples to calculate the volume needed to spike, in a way that would show expected trends in EPO doping, requires elaborate and deliberate sabotage.
Finally, just because Ashenden's ideas about spiking samples seems a bit flaky, I do not believe it detracts from his other opinions. That's because he is absolutely clear that he is unsure of what he is talking about. He starts the discussion with "I honestly don't know how you could even attempt such a process... and ends it with "...I am not confident I am correct!...." When he is not sure or the topic is outside his expertise he says so. IMO this adds credibility to points that he says he is certain about, rather than detracts from them.