Is that true? Is it not legally compliant?
Yes and no. It wouldn't be a legal sign on a highway, but the piddling small print at the bottom claims that the busway is a permissive path. I'm pretty sure that the vast majority of the busway is actually a Bridleway for cycling and only a permissive route for buses. If someone can tell me where exactly that sign is, I'll tell you whether I think it's permissive path or not.
and anyway - genuine question - can you have a speed limit for bikes
most "analogue bikes" don't have a speedometer
On a permissive path, the landowner could set whatever silly rules they like. The main enforcement action they could take is to ask you to leave by the shortest route and try to stop your trespassing. I don't think they could fine you or much else.
On a bridleway or other highway, there is no law enabling a speed limit for bikes, whether or not they have a speedometer. The speedometer thing is often mentioned but it's a red herring. It might have once helped convince MPs not to pass a speed limit law for pedal cycles, or that it would be unreasonable, but it's not itself any obstacle to such a law.