The sarcasm is not necessary
It was not sarcasm.
The fact is assumptions are make about postal history most of the time. Unless one is personally involved in the creation or transport of an item, the item itself does not prove much of anything. When that item is separated from those involved in the creation and carriage of the item only assumptions can be made.
One common assumption is generally postal employees properly carry out their duties. I will use classic FDCs and EKUs as an example. Each is date dependent with the assumption the postal employee was not backdating an item. Yet mistakes happen. A nine can be inverted and appears as a six, thus producing an item three days earlier than the EKU which was the 9th. Likewise, could someone back date a FDC the next day to get the FDC date even though the clerk or postmaster did not want to open the office on a holiday such as New Years Day. Short answer, "Yes." For those who have an interest in 12-7-1941 Hawaii postal history, it is well known that there are many 12-7-1941 dated items which were back dated, especially on military post office matter.
I have the earliest known dated parcel post usage which is tied on a post card dated 8-16-1912. That is months earlier than the 12-31-1913 dated usage of a 2 cent Parcel Post mailed on an envelope to none other than Carrol Chase (has cert circa 2010, but at the moment I don't remember if PF or APEX). Now the 8-16-1912 dated usage is a very clear strike but since the stamp design was not even created by 8-16-1912, what gives? Well, the post office of mailing was a seasonal resort area small post office where the clerk or postmaster failed to change the year of 1912 to 1913 for the new season. Here the facial evidence establishes the dated postmark while "real and official" could not have been applied that day.
Now I excluded "recent" FDCs since those can be dated for the Fist Day date but be applied a day, weeks or months after that date. Yet modern dating errors still happen as on a registered package I sent one night at about 11:30 PM using a night window in Oakland BMF that was not usually open. The other window was the normal late night window. Well the clerk pulled open the drawer, moved items around and grabbed the round dater for registered mail and neatly beat the heck out of the package as required by regulations. She then showed me her work...and I mention that it was August not still April. She, clearly shaken by the error, then corrected all of the dates, including the one on my receipt.
As to modern FDCs, buying the stamp on the FD and mailing it from another location on the same day gets a FDC which was for certain (except for human error) actually cancelled on the First Day such as this recently acquired example:
Now in keeping with the theme of the OP, do you think the sender drove or flew the 346 or so crow-fly miles to San Francisco from Pasadena, California after buying the stamp? Does it really matter? As for the envelope, transit took eight days and thus it did go by airmail to China at least part of the way.
No, John Becker, no sarcasm was intended. I truly wanted to shed a bit of light on the number of routine assumptions made in this hobby.