Intercepted, I noted the same thing and wondered about it. I meant to be thorough about checking it, but due to various distractions I did not make enough notes to be able to tell. Also the aircraft was VFR and had no ILS in the panel so I could not compare. However, the experience from flying the ILS was not particularly different from what I am used to, so I too suspect that the manual has it wrong. If the glideslope dots are 1 degree each you would need to go 2 degrees below ground level to see full upward deflection on a standard 3 degree glideslope, and that does not make much sense.
Both needles move in half-dot steps, an information not available anywhere I have looked. If that seems a bit coarse, the flying experience actually suggested it is adequate.
I did not find a way to order the SP-400 other than directly from Sporty's in the USA. Shipping was 10 days to Denmark and cost a modest $14.80. However, the 12V power cable is in back order.
I expect to be billed for custom.
Back to IO540's initial question: the headset adaptor looks identical to my 15 years old King's adapter. So if ICOM and the old King's match, the answer is yes. Might be industry standard.