but wouldn't capping this off and just releasing pressure through the steam wand on startup accomplish the same thing?

It would, BUT at a price. There are a few machines built today, e.g., Salvatore and likely many built in former years without vacuum breaker valves. The warm-up routine was as you suggested with the addition of opening the steam wand when you shut the machine off for the day.

However, as an operating machine cools, a vacuum would be formed within the boiler and this can be detrimental to parts not designed for same (boiler pressure gages, pstat diaphrams) should the operator forget to open the steam wand valve.

The "line in the sand" - actually the boiler water level line where dry steam turns to wet steam is not always clear. Wet steam is called "carryover" and self describes water particles mixing with the steam due to a higher than "proper" water level.

In reassembling the machine, you just might have unintentionally placed the water level probe a tad higher than before.

I will follow Eric good advise,

could also be a slow overfill, such as your 2 way solenoid is not completely close when ti should

also the longer style valve in relation to the standard anti-vacum always seam to spit more water out during warming up

also in some situation the heating element can Build heat faster than in some other (scale build up etc etc)
which will keep the anti-vacum in the opening/spitting state (before it build enough pressure to seal shut)
a little longer then it should
but this will not have anything to do with having a wet steam once the pressure is reached