The Guardian misread what Donald Trump wrote: "In Britain, more Muslims join ISIS than join the British army."
They haven't checked how many Muslims join the army and ISIS every year. Since ISIS has only been there for three years, Trump may be true.
To quote one of the comments:
"To obtain an accurate comparison, you would have to look at how many Muslims have joined ISIS and HM Armed Forces, not compare ISIS recruitment against current HMAF total. It would also be more meaningful to see if HMAF recruitment of Muslims has risen, fallen, or remained static over the past 10-15 years."