mammonista - as one whom dearly wished to build a series hybrid (diesel gen/electric drive) but was forced to the compromise of my parallel hybrid
(gasoline/electric dual drive) due to some of the difficulties above, I assure you it isn't "nonsense" as much as cautionary concerns, difficulties that you may or may not encounter when attempting a diesel-powered motorized bicycle, just about every one of which I stumbled into myself to some degree.
Are they 100% accurate? Of course not, there's bound to be an exception but without direct first hand experience all folks can do to help with this very much obscure and difficult to obtain engine class is pass along what they've heard, what they've seen others attempt and what generalities they're aware of regarding such... and they're not that far off the mark.
I would take it as well-meaning advice, no more then a "heads up" regarding potential problems, problems if you can circumvent so much the better and if/when you succeed you can happily refute to everyone's benefit.
I didn't give up on my diesel/electric for a gasoline/electric because it was "impossible" BTW, rather that acquiring a compliant diesel in the size/weight/displacement combined with the requirement of commonplace repair & replacement parts proved to be enough of a hindrance to the project overall that I decided to forgo the diesel until a suitable motor could be found...
...so good luck, we're all just hoping for the best