"I think this sort of presentation isn't likely to convince linguists." Speaking as a linguist... you're right, it doesn't convince me. Nearly all languages have more sounds (phonemes) than the symbols shown in most of those infographics--more consonants, even (many early writing systems only represented consonants). All languages have more distinct syllables than any of those symbol systems (many early writing systems were syllabic). And obviously all languages have far more words (or morphemes) than any of those systems.
What would be more convincing? A sequence of a few dozen symbols in some particular location, and likely to have been written at the same time (rather than centuries or millennia apart), by the same person (so if there were handprints, the handprints would be the same person's hand(s)), where the number of recognizable symbols is twenty or more. I don't say that would be all that would be needed to convince linguists, but it would be a start.
Can we say "language representation that is not a transcription of a spoken one"? A sequence of symbols could have meaning without mapping to sound, and taken at face value the separation on the map would seem to imply that any spoken language could have evolved dramatically out of sync with any physical representation. I can't think of any reason to think that - if you assume a phonetic interpretation - a symbol shared between North America and southern Africa would be pronounced at all similarly in both locations when the marks were made. The distances alone argue against a phonetic interpretation to me.
Do hobo signs count as a language? That seems intuitively much closer to what this might be. How much structure do you need?
"Can we say 'language representation that is not a transcription of a spoken one'?" Of course: sign languages. But sign languages have even more "phonemes" than spoken languages do, so this doesn't help the hypothesis.
And yes, "A sequence of symbols could have meaning without mapping to sound": that's what hieroglyphic writing systems were, more or less. Again, more distinct symbols, not fewer.
"Do hobo signs count as a language?" Depends on how you define "language". As most linguists would define language, though, the answer is no. All normal spoken or signed languages have oodles of structure (grammar). Pidgin languages probably do not, but that's just the first generation or so, after which they gain structure and are technically creoles. (Some creoles have the word "pidgin" in their names, like Tok Pisin of New Guinea, and Hawaiian Pidgin, but they're complex enough to justify the term "creole.")
Odd that you say “nearly all languages have”, which would include that other languages you yourself can think of, do not have “more sounds[…]symbols…”
The problem I have with “experts” when it comes to this kind of dogma challenging thing is that they are usually extremely biased and limited by that dogma.
Let me put it this way, if you saw those symbols on Mars, would you not consider them a form of communication or language. Ironically, to me at least, the limited ands relatively consistent nature of the symbols itself actually qualifies it as language, not disqualifies it.
Two languages I can think of that have approximately the number of phonemes that some of those sign lists have are Hawai'ian (at least 13 phonemes, depending on how you analyze vowel length and diphthongs) and Central Rotokas (11 phonemes). But that low a number is extremely rare among languages.
"...if you saw those symbols on Mars, would you not consider them a form of communication or language" Communication, yes. Language? Knowing nothing about Martians, I'd have to withhold judgement. On Earth, language is much more than just communication.