Do rips of commercial discs read as 16bit? Just curious. You may also want to look for a program called LeeAudBi.exe for a more detailed look at what is in the frame headers. My guess is that the Surcode and eac3to encoders just don’t set the necessary bit in the frame header to describe it as a 16bit source. This source lists the contents of the frame header, and shows that there are three bits describing the source audio bit depth.
https://wiki.multimedia.cx/index.php/DTS#Frame_format
My guess is that your encoders just leave it set to 000 or 111, one of which probably corresponds to 24-bit. This is a total guess though. Remember that this is just a description of the source bit depth. Your other sources are correct in saying that bit depth is not really relevant to lossy compression codecs. The decoding of the audio is not necessarily effected, except that some decoders may try to match the bit depth of the decoded output to that of the source.
other info here: