The committee said he lied while under oath in giving evidence to the committee. If proven this is perjury. That is not held under a convention, this is law.
If any person lawfully sworn as a witness or as an interpreter in a judicial proceeding wilfully makes a statement material in that proceeding, which he knows to be false or does not believe to be true, he shall be guilty of perjury, and shall, on conviction thereof on indictment, be liable to penal servitude for a term not exceeding seven years, or to imprisonment . . . F1 for a term not exceeding two years, or to a fine or to both such penal servitude or imprisonment and fine.
The Commons Privileges Committee is not a judicial proceeding. It operates according to the rules and conventions of the UK Parliament. It is separate from the legal process, because only parliamentarians can make decisions about issues of parliamentary privilege. (Verbatim definition from parliament.uk above) So whilst I’d love to agree with you, the law cannot be applied because it is not a “judicial proceeding” by definition.