The reason Vne can be shown on the ASI as an IAS is because in that design configuration, the aeroplane will never get anywhere near Vne in level flight due to the power self limiting properties of the engine in relation to altitude (where TAS departs IAS considerably). So in a TB20, with say a Vne of 180 kts IAS (just made that up by the way), likely at FL200 you will show 100 kts IAS. This 100 Kts IAS will be less than the Vne at 140 kts TAS.
However what Vans were warning about, is those who insist on taking an RV6 (for example) and changing out the IO360 with say a TIO360.
Now the aeroplane has a turbo charger and can hence develop much more power at high altitude, which = > IAS. Even though s(he) may be travelling at 150Kts IAS which is well below the indicated Vne of 180 kts (made that up too) the TAS is 210 Kts which is > Vne by 30kts !
Gliders are different animals obviously as they don't have an engine to deliver less power at alt, which is why the obviously have Vne tables.