No, that is not true.
It wouldn't make sense for SoapUI to have such a restriction since it's open source; so anyone could fork SoapUI and remove the restriction from the source code.
This confusion might be because there was a bug limiting the number of threads in SoapUI 4.5.0 (both Open Source and Pro version). This bug is fixed in 4.5.1
Source/disclaimer: I'm a developer working full time on LoadUI and SoapUI.