Its still the same issue, We have Mathcad 15 50 with MS office 2010 and also with MS office 2013 the same issue.
I know this isn't the solution you're looking for, but a very long time ago I learned to completely stay away from APIs...for this very reason - version compatibility issues will NEVER go away. Yes, they are "really cool" and you can create awesome ties between two codes, but at some point one of them will update and you'll be out-of-step until the other catches-up. The only way around this is to remain on the two versions that play nice together (if they ever did at all) until the next, newer versions that work together are available to install. This approach is easy if you're working alone and control the software loaded on your computer, but almost impossible if you're at a large-enough company.
If you work at a company where your Mathcad files are, or might be, used by others, you could consider linking one tool into the other if you are making a "one-off" tool for yourself...but that normally turns into something cool that others would want...and then your nightmare begins. The only capability I've ever continued to use for any kind of link between Mathcad and Excel was a Mathcad function that reads/imports Excel data in.
Bottom line is - stay away from APIs...they will cause trouble at some point that is not worth the original effort.
My 2¢ - Cheers!