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I tried asking this on Skeptics first, but apparently there's too much law (interpretation/opinion) and not enough fact in this issue.

Several conservative/right-wing sources says that if the US Senate were to ratify USMCA (Trump's renegotiated NAFTA), they would basically ratify UNCLOS by implication as well. (For more context, the US has signed but not ratified UNCLOS/LOST.)

E.g. the John Birch Society says:

Trade can be done without these many-paged deals. But the USMCA isn’t about trade. It is all about the establishment of regional and world government. This dangerous deal would in effect ratify the Law of the Sea Treaty -without any Senate vote on it.

A more detailed article in Canada Free Press concludes with the same:

In article 24.18, Sustainable Fisheries Management, regulating “marine wild capture fishing,” USMCA agreement subordinates the United States to U.N.’s international authority and its many organizations. [...]

Sustainable fisheries must abide by the United Nations Convention on Law of the Sea (UNCLOS), the United Nations Agreement for the Implementation of the Provisions of the United Nations Convention on the Law of the Sea of December 1982 relating to the Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks, the FAO Code of Conduct for Responsible Fisheries, and many others. (USMCA, art. 24.18)

A.J. Cameron stated that the “same people in the Obama Administration who crafted the TPP also crafted USMCA. USMCA back-doors many of the tenets of the reprehensible trade agreements to which we were told by politicians that we would not become a member.”

[...]

The European Union and 162 countries have joined the Third United Nations Convention on the Law of the Sea (UNCLOS III) which was adopted in 1982 and now called simply The Law of the Sea Treaty (LOST). [...]

Under LOST, any kind of maritime dispute, fisheries, environmental protection, navigation, and research must be resolved under this treaty through mandatory dispute resolution by the U.N. court or tribunal which limits autonomy. But disputes should be resolved by U.S. courts.

When Congress approves the USMCA agreement, the Law of the Sea Treaty (LOST) will also be ratified through the back-door, by including it in the USMCA. Which senator is going the read this massive bill?

Is this interpretation (UNCLOS-wise) of the implication of USMCA ratification correct?

While I haven't read the full agreement (trade deals aren't exactly riveting), there seems to be a few gaps in the arguments given. Here's Article 24.18(3) (PDF link):

Each Party shall base its fisheries management system on the best scientific evidence available and on internationally recognized best practices for fisheries management and conservation as reflected in the relevant provisions of international instruments aimed at ensuring the sustainable use and conservation of marine species.

In my opinion (since I can't find too much independent commentary on this point), the US would only be required to abide by UNCLOS insofar as it relates to fisheries management. UNCLOS covers a much wider range of topics than just fisheries. There appears to be no commitment beyond that. On the other hand, there's no dispute that UNCLOS is one of those "international instruments," as it's referenced in the footnotes to this section.

Also, since the US is not party to UNCLOS, dispute resolution would be through the panels established by Article 31 (PDF link), not through UNCLOS tribunals which might otherwise be possible between Canada and Mexico, as per 31.3, for example.