Importing TCX to BaseCamp fails with "invalid GPX file"

When trying to import .tcx files, e.g., downloaded from web Connect, results into an error message "<filename> is not a valid GPX file and could not be opened." Well, that's very true, since it's a TCX file. However:

- I have a recollection that TCX import was supported, and the BaseCamp documentation would support this.

- Even the file type association of TCX is to BaseCamp.

- The "File->Import into..." does not show TCX files as supported..

If I have messed up something or trying to import the wrong way, I would appreciate the problem being pointed out.

Running BaseCamp 4.8.10 on Catalina.