I only have the fixes-3 branch at hand... But it is very similar to 3.6, and should be the same for the HL.
I tested my above code snippet, and "readonly" was not highlighted.
So yes, that is fixed in 3.6
I do recall there were several fixes about keywords/modifiers in various expressions (including properties). I don't know which was fixed in exactly what version.
Afaik there is some Pascal dialect / $mode in which a property can have a readonly keyword somewhere. I don't recall the details. (All I could find is a comment in codetools). So that is, why it was considered at all for HL, but wrongly so if it was an identifier.