XMPlay says "XM?" when it detects that the XM file was not made in FT2.
Most of the time it's not possible to detect what tracker an XM was made in, but some old MPT had a small difference in the header - it said "FastTracker v 2.00" instead of the normal "FastTracker v2.00". I don't think the new MPT versions do this anymore, which is a pity because it atleast made them detectable
... XMPlay also says "XM?" when there are more than 32 channels, because FT2 does not support more than 32 channels.
XMPlay does a few things differently for the non-FT2
detected files, to match some things that MPT does differently to FT2.