First off, I have not seen any specific information regarding the contents of the ssl_version field in ELB logs, which would be the correct place to authoritatively answer what the ELB logs mean.
That said, in this kind of context the string TLSv1 should reasonably be interpreted to mean TLS version 1.0.
In your particular case, this would imply that those clients do not seem to support higher versions (because they would be expected to prefer and use those whenever possible).
It is worth noting that strings like SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3 especially when written just like this (and sometimes minor variations on the theme, depending on the software involved) are often not free-form text crafted with the exact current context in mind, but rather protocol identifiers used in the underlying software.
As for the potentially confusing TLSv1 identifier widely used for TLS version 1.0, I think what we can read into this is simply that popular implementations (like openssl) were kind of caught out by the change of versioning policy that became clear only when TLS 1.1 and the following versions were introduced much later.
Ie, SSL had been versioned 1.0, 2.0, 3.0, but since the protocol changed names to TLS the versions have instead followed the pattern 1.0, 1.1, 1.2, 1.3. At the time when TLS 1.0 was first introduced, I guess that the assumption simply was that the next version would be 2.0 and that continuing the same pattern of simplified identifiers like before would continue to make sense.
Sidenote:
It is correct that curl has command line options that makes clever use of --tlsv1 meaning something different than --tlsv1.0, but the question is not about how to instruct curl how it is allowed to connect, but rather to parse what a log entry specifies as the exact version a certain connection used.
