Yes, this is true.
I'll try to explain: In their work which forms the basis for Spectre's stb analysis (see
http://www.thekunderts.net/ken/docs/c%26d2001-01.pdf), Mike Tian and his colleagues (Ken amongst them) chose to define the loop gain with a different sign than it is done in most engineering textbooks. Please refer to the paragraph following equation (3); most textbooks define T(x) as the loop gain, Tian however defines it as -T(x), corresponding to a phase difference of 180 degrees with respect to the more usual definition. This sign convention was carried over into the implementation of the algorithm in Spectre.
One of the reasons for the confusion about the "proper" sign for the loop gain is the fact that Hendrik W. Bode, who introduced the concept in his 1945 book "Network Analysis and Feedback Amplifier Design", never really used the term "loop gain" there, but talked of the "return ratio" instead. Since then, most authors have chosen to define the loop gain with the same sign as Bode's return ratio, some however (e.g. Tian) have chosen differently.
The unusual sign convention used by the stb analysis also leads to a somewhat funny inconsistency between Spectre and the calculator functions of Analog Artist. These calulator functions expect the more usual definition, so that you have to plot phaseMargin(-getData("loopGain" ?result "stb-stb")) or gainMargin(-getData("loopGain" ?result "stb-stb")) in order to get the correct result.
The values for phase margin and gain margin which are reported by the stb analysis are of course also correct because they take the definition of the loop gain into account. You have to note, however, that the stb analysis reports a positive dB-value for the gain margin of a stable circuit, whereas the calculator function gainMargin() reports a negative dB-value. Both report a positive value for the phase margin of a stable circuit.
If you have access to SourceLink, you can also take a look at
http://sourcelink.cadence.com/docs/db/kdb/2004/May/11138588.html where they try to explain the issue.