It’s neither true nor false.
If trying to lump every quantifiable skill into one aspect, then 3s would technically require more skill [e.g., teamwork, communication, precision, finesse, playing around the given meta, etc.], but solo shuffle certainly requires more individual accountability/skill and in more unpredictable/less scripted scenarios [e.g., having to rely more on yourself than partners, having to be a playmaker to climb, adapting on the fly to a lot more comps and the generally unpredictable nature of random partners, having to play with and against everyone in the lobby, not having voice to coordinate kills and cooldowns, etc.].
You’re definitely right in that participation skews the numbers, which is largely due to how much more accessible shuffle is (I’m not necessarily certain if shuffle would be more popular if 3s had a solo queue button), but in the grand scheme of things, the skillsets required to succeed in each bracket have a lot of overlap.
In other words, if you can get rank 1 in shuffle, I’m pretty confident that you can get rank 1 in regular 3s, and vice versa (if you can find two others of comparable skill and have a decent comp). Similarly, the difficulty required to get 1600-2400 in shuffle vs 3s I would imagine is comparable as well (not accounting for the extra difficulty in finding partners and building synergy).
Anyways, thanks for putting in the effort to collect and present the data 