I have some good news for P1-owners! I got the skew compensation to work on my P1S, firmware 01.06.01.00, by adding M1005 S1 before setting the skew with M1005 I[...] at the end of the start gcode. To me, this implies that skew compensation is disabled by default? Just to be sure, I’ve also added M1005 S0 at the top of the start gcode, although I don’t think it’s needed unless I’m saving the settings with M500.
One thing to note: during testing, I was using 20° of skew (M1005 I0.34907) to quickly see if skew compensation is working. With the above method, the print head ran into the glass door at the end of the purge line, so be careful when using large skew values! No issues so far for me (-0.17°, i.e. M1005 I-0.002967).
Lastly, I was using the Califlower Mk2 with the accompanying spread sheet for calibration, which reported -0.17° of skew. Setting -skew in radians as a correction, i.e. M1005 I0.002967, and retesting showed an increased skew of -0.30°, while using the skew directly in radians, i.e. M1005 I-0.002967, gives me close to 0 skew. Not sure if I misunderstood something, or how interesting this is to others; this is mainly for me to remember the procedure for next time 
If I find the time, I may retest with the calibration square and the XY Skew calculator linked above and compare.