Actually now you mention that about
5994001 I have seen this i.e. class 599 no, 4001 etc, proposed before on a forum*** somewhere but it was long after both the 994 and 599 so called 'classes' were known and was dismissed as a retro explanation (like how a backronym works) and not correct. Who knows though.
I do suggest the thing here is understanding there are grey areas between the blacks and whites of what is a class, what is a set of vehicles, and what is a vehicle number, and why or how certain non standard movements or traffic need to be input to systems maybe with work arounds - and trying to fit EVERY example into neat trainspotter type OCD

categories for one's P5 ABC Book Of Numbers

will fail.
Besides, we need anomalies like this to generate web wibble and support internet bandwidth consumption this sustaining the ISP and IT industries.
I am pretty sure the same thread had someone suggesting that Eurotunnel so BoBoBo electric locos "must" be class 99 in TOPS on the 9 (for electric loco) + 9xxx number despite that they never turn a wheel on GB main line metals; that one sure was dismissed out of hand. But you can see both where some of this dribble comes from and the OCD need to neatly categorise everythings..