If you think that's corporate drivel at its worst, you haven't seen much corporate drivel. What's so bad about it? She is diplomatically trying to say, "Hey, we don't have the money or resources necessary to support Thunderbird, we're probably going to abandon it".
Open Source may be well and good but the engineers at Mozilla still have to earn money for their families. Look at the Corporation's Financial Statements (https://static.mozilla.com/moco/en-US/pdf/Mozilla_Audited_Fi...) - they don't exactly show a company flush with cash that is pulling support for a product because they don't like it or something.
I like Enigmail though, just as you do, and tried introducing my family to it. Sadly that didn't stick :/