First thing first, which treaty are you referring to?  This is important as different treaties have different criteria and limits for the exemption.  Income paid to teachers is not exempt unless those conditions are fully satisfied and the income is within the limit.

Also, Article 21 usually refers to Students and Trainees, whereas Article 20 governs Teachers.

Second, you are filing a 1040NR.  I suppose your client did not satisfy substantial presence test by default.  You should also consider whether your client should qualify as an exempt individual as this affect not only your client's resident status in 2020 but also FICA liabilities.

You also mentioned your client receive a W-2.  Educational institutions are generally very up to date on the application of domestic tax law and treaty exemptions on J-1 (as well as F-1) visa holders, including certificates that need to be submitted for exemptions for withholdings.  If they did issue a W-2, that is an indication that something could be amiss.

It is correct that the amount from Line L will flow to 1040NR Line 22 for information only.  That number does not add to Line 23 or anywhere else.  If you can provide a redacted screen capture or the numbers shown on the return, we can take a second look.

If I understand your message correctly, you are also looking at exempting your client's compensation from CA tax.  CA has it's own residency rules, which do not follow the IRC, and does not recognize US income tax treaties.