I do not know if that is entirely accurate, and this may be jumping to a conclusion that possibly is unwarranted under the Guide to Safe Scouting, which is the text by which BSA has informed all adult leaders of the rules to be followed for the Scout's, the BSA's, and the Adults' protection.
The purpose of two-deep leadership within Adult-Scout communications is to ensure that another adult is, at a bare minimum, made aware that a communication or interaction between an Adult and Scout has occurred. It is intended to put another responsible adult on notice and to be aware of this occurrence. Accordingly, sending a piece of mail addressed to a Scout at his or her home address puts that Scout's parent or guardian on notice that a communication has been received made from another adult. If the parent/guardian wishes, he or she could open the mail for the Scout and read the correspondence—or, better yet, read it with the Scout.
The possibility and opportunity is there, and the parent would be appropriately "carbon-copied" on the mailpiece, just in the same way as e-mail, or sitting in on a phone call over speakerphone, or sitting in the back of a videoconference in Zoom. Perhaps there is a rule in the GTSS directly on point, but as far as my YPT training and GTSS knowledge shows, I see no issue. Thus, I see no problem with a SM sending a handwritten card to one or more Scouts under the example the gentleman posed.
As for the rest of this thread, I disagree that the requirement of two-deep leadership for video-conference meetings is in any way detrimental to the goals of Scouting, and I must respectfully question those leaders who would be vehemently opposed to such a requirement (i.e., those who would call such things "stupid" or "nitpicking").