I have no idea about any Jack Casady/Phil Lesh/Alembic connections. I can tell you that anyone who says Pyramids have changed since the old days either never played 'em back then or doesn't have a good memory.
They were never low tension, just average for their gauge sizes. Any small gauge set on a short scale bass would feel relatively soft. They never actually advertised the actual gauge sizes but the short scale set was something like 40-95.
Pyramids are nothing like TI flats and never were, either in tone or feel. I don't think Pyramids are exceptional, i.e. they're good but not worth the extra money unless you need them for a special reason like playing what your hero played on a certain record. They're full range medium tension flats, unlike the TIs, which are flabby and midrange heavy (I don't like TIs, in case you can't tell).